KDE 3.5.1 isn't released. But it's been tagged, which means an announcement may be in our near future. I svn'd the sources and built them over this past weekend and thought I'd give fans a bit of a sneak peak.
KDE 3.5.1 is a bug fix, security and update release, or rather, a maintenance release. There aren't too many new features to speak about, in fact I haven't eyeballed any. But I can report that a couple of annoying bugs I had are now cleared up and the overall performance has improved yet again.
Is it just me, or does this KDE get faster each release?! Logging in shows no real improvement in speed really distinguishable by the human eye, but the applications are clearly more finely tuned. On my AMD Althon XP Barton 2800+ with 1 gig of ram, applications open almost instantaneously. After the initial starting of konqueror, subsequent events are indeed instant. Things are more stable here and a few bugs are worked out.
According to the Changelog , some of those fixes (so far) include:
- Improved performance under some conditions on the networking classes.
- Mimetype detection: add more "magic" patterns to recognise shell scripts
- KJS: avoid buffer overflow on decoding utf8 uri sequences
- Turning off tooltip doesn't work (fixed)
- Don't crash on logout.
- app started by kmail does not show in kicker (fixed)
- Desktop icons move down at each login due to child panel at top of screen (famous moving icons fixed)
- Performance improvements:
- Add caching for getElementById
- Much faster appending of options to selects, and clearing of selects
- Simplify dirty region computation, to speed up cases of lots of small updates
- Small improvements in efficiency of URL handling in image loader
- Fixes reply buttons on Yahoo mail
- Do not lose server-specified encodings when going back in history
- Hidden field is not sent (Fixing OSNews.com)
- Konqueror consuming all memory when printing specific web page
- Various crash fixes
- Performance improvements:
- Don't crash after selecting variable
- 2 Tools menus when embedding KPDF in Konqueror
- Fix initializations for desktops >4 when using DCOP calls
- Incorrectly restored window geometry upon leaving fullscreen mode under certain circumstances
- Fix memory corruption bug causing crashes with recent glibc libraries
- Fix incorrect initial volume setting when using aKode as output
- Corrected list of file types that JuK supports
- Fix "Album Random Play" feature to also use Artist information when choosing the next track to play
- Added musicbrainz 0.4-support
- Fix disconnects/crashes after connecting to a Yahoo webcam
- Don't send picture information packets to Yahoo buddies when connecting into invisible state
- Don't crash when deleting several contacts that are in severeral groups
- fix initialization problem with some modems (Qualcomm 3G CDMA)
- support higher connection speeds
- KMail crashes while inserting files (bug 111383)
- KMail composer crashes when inserting file (bug 108063)
- KMail crashes when inserting files into email messages
- Crash when applying pipe through filters
- HTML formatting is lost when saving in drafts folder
- Fix problems with keyboard navigation getting stuck
- Fix parsing of Atom 1.0 feeds with escaped HTML in it
- Fix article order in Combined View: sort by date
- Do not open binary files in the HTML viewer but externally. Fix "Save Link As" for binary files
- Allow to remove a local theme
- Enhanced itemDropped-func with x,y-value
- locale in python themes
- Applets appear on top after switch desktop
- Non-intuitive theme removing in Theme Dialog
- Full changelog 
One of the most welcome fixes for me was the Yahoo reply bug. I first noticed it in 3.5.0 well after my published article. I don't really use the web interface too often, but when I want to, I want to. At first I thought it was just me or my setup, until I started seeing others posting about it. I tested it quite extensively this evening and can report all is well. I'm glad they fixed that.
The kwin bug took an interesting form here on my (tux)machine. Again, something I didn't notice until after I published my last article, but some windows, like the 'open file' dialogue, could not be adjusted. It was small and it was going to stay that way. Now I didn't think too much of it and did not blame kde too much as I commonly run KDE apps from within fluxbox. I thought perhaps my bug was a result of that. But in any case, I do not have that problem now.
Another bug I seemed to suffer with was this "empty trash" freezing up the system. The process would hog all the system resources and hang forever. As running fluxbox or deleting from the commandline is my more common practice, this was not a showstopper for me - except I had a trash can full of stuff that I could not get rid of. They'd been there for awhile. Prior to Trash becoming an kioslave process, one could just delete the files from the Trash directory. I often wished they had left it a directory. I had other uses for Trash as a directory as well that now is history. But oh well, progress progresses. At last I was finally able to dump that Trash can full of old garbage. I was most happy to reclaim that ~400mb of space.
UPDATE: As it turns out, Trash is now stored at ~/.local/share/Trash
I also had trouble in Kmail in previous version when trying to expire messages in a medium-sized folder (containing a mailing list). I wanted to set it an expiration date of 7 days. However, this action would crash kmail (kontact) every time. I can now report that issue seems to be resolved as well.
I've had a few crashes of Konqueror under 3.5.0 and they do seem decreased in numbers since my fresh install of kde 3.5.1. Any crash is too many, so of course any improvement in that area is most appreciated.
That ugly kicker crash on logout bug is gone as well. I know many many people will be glad of that.
The popup reporting the suppression of popups is still annoying me quite a lot tho. Hasn't anyone found a way to turn that off yet?
So, no new eye candy or new features, just a faster, more stable, less buggy, more productive desktop environment will soon be available for your enjoyment. I've been running kde 3.5.1 for a few days now, and I haven't found any new bugs or problems as of this writing. Of course the fix for that nasty buffer overflow  so widely publicized around the globe is included at no extra charge. In fact, with the release of 3.5.1 expected so soon, it may make it to the shelves before your distributor puts out fixes to 3.5. I know of one vendor who as posted some early packages of 3.5.1 for preliminary testing. Overheard reports of them are quite favorable. Of course, we're all just really counting the days until 4.0!