KDE4 user? Been having problems with the DCOP and DBUS servers? Do your K-applications no longer launch from the graphical user interface? Do they sometimes not even launch from the command line? Have you got references to "file retriever failed: 149" in your .xsession-errors log?
Well, you're in luck. I've been having this problem recently - it's especially annoying as things like KNetworkManager and KPowersave can be affected, although non-KDE apps still work (except, of course, when they have to integrate with a K application in some way). It's associated with KDE4.1 in some way, as KDE3 apps still work, including the KDE3 versions of the same apps. But I'm not any more. Trying to fix the problem, I decided to have a look in the /tmp/ directory, and what did I find? North of 250MB worth of worthless cruft, old prefetched web pages, prefetched ads from old web pages, blank cache files, that sort of thing.
So I removed it all; just to be vindictive I ran the KGPG Shredder function through it as well. And then I moved on to the /yorksranter/tmp/, which was also stuffed with crap. Upshot? Everything works again, faster and better than before. Presumably, the stuffed /tmp/ caused KLauncher to time out before it got something registered somewhere. 250MB is a lot of cruft, but it's not that much; I think the problem was caused by the number of directory entries rather than their size. Perhaps I should set up a script to flush the e-crapper automatically.