Gmail made some very minor changes to their website (they renamed their Html-classes). Unfortunately, it was enough to break Folders4Gmail.
Please install the latest version of Folders4Gmail to make it work again.
Version 1.41 also works better with right-to-left languages like Hebrew and Arabic.
For those of you using Better Gmail 2, I emailed lifehacker.com about the update. So there should be an update for Better Gmail in a couple of days.
Sorry for the trouble. Unfortunately, userscripts are very vulnerable to such small website changes.
[Update March 5th, 2009]
The update for Better Gmail is available now