iCloud backup stopped working

I noticed over the weekend that iCloud had not completed a backup since Dec. 18. When I go to Settings -> iCloud -> Storage & Backup, it says "There was a problem completing the backup. Please try again later." If I tap "Back Up Now" I get a progress bar that never moves, and it says "Estimating Time Remaining" and then it fails again with the same error message.

It says I have 731 MBs available of my 5.0 GBs of storage. If I look at the current Backup in "Manage Storage" it says it was from 12/18/11, and the backup size was 2.0 GB.

Help! Any ideas?