Same thing happened to me last night. It had happened to a friend of mine on his 1st Gen using 2.0 firmware too. I googled around for it and it seems to be a common problem.
One thread
here suggested downloading something so you have to re-enter your iTunes credentials. I did this via some updates and now 80% of my apps are working... except the two I care about, Pandora and Facebook. Haven't tried to delete and reinstall yet, but I'd rather not bother with a restore. I hope Apple addresses this sometime soon as I couldn't find an official word on it.
EDIT: Deleted and re-installed facebook and it works correctly
