I came across this via The Atlantic's print edition, not via any of the many blogs I track. Odd.
Carbonite Online Backup is XP only, but the price is right. $50 a year for "unlimited" (really?) backup.
If it were OS X as well I'd have signed up by now, but no matter -- the $50 price point is the key. At long last, online backup is going mainstream. It's the only way to go, but expect restores to be unbelievably slow and tedious for sometime. It can't be the only backup solution, but it's a good companion to backup to an onsite external hard drive.