Also, interestingly enough, when it reports the failure, it gets the date requested wrong. I just tried again and got a backup failure, but five minutes after I tried to run it, the failure message said that the date requested was "17 Aug 2012, 20:05 GMT-04 (2 hours ago)". The time is about right, but it was a few minutes ago, not 2 hours ago.

I can confirm it's not working for me either. Mine failed after a few minutes and was reporting the time correctly.

I did run a successful backup on the same site yesterday. The file was time stamped with 20120816_2244UTC, so it was working about 28 hours ago.

To make sure it wasn't a problem with the size of the site, I tried another backup on a site with just 97 pages (my other site is over 2400 pages and I added over 500 of those today). The backup on the small site failed too.

My backups are failing too. Date and time are correct.
I even tried to backup a site with just 5 short pages and it failed too.

A backup just worked for me!

I do have some more information on the time anomaly, though: When I re-run a backup by clicking on the "Create a new Backup" link on the page which is displaying the "backup failed" message, the time that shows up is the time since the first attempt to do a backup. 12 hours+ in this latest successful attempt. But if I click "Backup" in the "Manage Site…" column, the time gets reset and is correct.

It should work fine now, please re-report if not.