A couple of things you can do, first is clear the queue in installer. If you had a single program that wont completely install it will stay in the queue and every time you try to install something else it will start with the bad install. If that doesn't work, refresh your sources. And finally make sure that you installed the latest community sources showing in Installer.
I saw quite a few ppl sayng that installing the bsd subsystem did the trick, I also had that problem but only when I tried to install video recorder from mmi source but when I tried to install it from the developer source it worked so I think that the one from mmi had some problems anyway hope this helps and bsd subsystem is a musthave on the iPhone
Well I got a busy weekend, so I'll tell you in case I don't get back here... I had the same issue you were having, and it had to do with the new Springboard.plist that Intelliscreen installed. It was not allowing stuff to be installed.
I SSH'd into the phone and deleted "/System/Library/LaunchDaemons/com.apple.SpringBoard.plist" and renamed the "com.apple.SpringBoard.plist.orig.preIntelliscreen " file to the original name. You'll have to reinstall Intelliscreen then, but this worked for me. Perhaps it has something to do with your situation.
if you don't have intelliscreen and you still have the problem then i would say you might want to look at you disk space. you might be running low. you could delete one of your apps and then download bosstool. then you can follow the intrucstions that are give on the app to free up some disk space.