Backup? Um... No.
I'm not sure it's in any way accurate to call Ubuntu One usable for backup.
Ubuntu One synchronises multiple PCs, and synchronises your content into the cloud for access anywhere. But if you delete or overwrite something accidentally on your machine whilst it's connected to the internet, then a minute or so later it's toast on Ubuntu One as well. That's hardly a good backup solution...
It seems to me that you've fallen into what I like to call the "RAID trap" - people buying RAID arrays often mistake RAID 1 or RAID 5 for backup, when in fact it's merely providing redundancy against hardware failure.
The bottom line is that if your laptop died, Ubuntu One would keep your data for you. But it can't protect you against your own stupidity...
(And a clarification - I'm not railing against Ubuntu One, and use it myself for syncing machines - but I wouldn't use it for backup! But it's a good sync service and it's looking promising in terms of projects using it - for bookmark sync, data sync, configuration sync, etc., which is why I'm a paying customer of Ubuntu One.)
Dropbox is slightly better for backup - it holds previous versions for about 7 or 14 days (I don't recall which, exactly) and they offer an option for subscribers called "Packrat" which keeps previous versions of files indefinitely.
In the end, I picked SpiderOak for backup. They seem to be the most secure, and give me 100Gb for 100 dollars a year, which is a pretty good price. It has multiple versions, and is cross-platform with Win, Mac and Linux clients. I was surprised not to see it mentioned here.
For local or local-network backups, check out the rdiff-backup package. Rsync and diff, combining to give space-efficient backups - even to SSH hosts across the network. I've had no problems with it.