Installed 270.51 @lunchtime then noticed a bit later the folding icon had the exclamation mark on it. Checked the log and said 5 failed attempts at starting would pause for 24 hours! I reisnstalled F@H manually cleaning all data up too, reinstalled and the same happened again. Was about to give up but did some googling and confirmed that 270.51 beta may stop folding@home GPU clients to fail. So just posting this in case you get this problem then you know what caused it. I reinstalled 267.85 and I have it running again Just to reiterate, this does not happen on all nvidia cards. Happened on my GTX430 though.
I've been using that driver on a system with a GTX 275 for about a week and no problems. This was installed on a fresh install of Windows 7 64bit, not upgraded from a previews driver version.