How much data are you talking about? Online backup is slow and can get expensive fast. I don't use online backup for my SFX Library, because it is just too damn big. Instead I back up to hard drives and have 2 offsite backups (at 2 different locations).
I do backup my current work online. I've tried a few services and currently use Dropbox. I found iDisk to be slower and more expensive than Dropbox. Amazon S3 is the cheapest solution, but since there is no cap on the amount of data that you can store there, I wasn't managing my data very well and was getting hit with rather large monthly bills.
With Dropbox I have 200 gigs (I pay annually for that). The assets for my current projects (PT Sessions and audio files, docs, reference material, ect) are kept in my Dropbox folder and are automatically (and constantly) syncing. Since I have a 200 gig cap, I have to manage my data as Dropbox starts to fill up. This means that I need to archive old projects and clean up unused audio from completed PT Sessions.
200 gigs works well for me. I may only run into issues if I have multiple projects running at once, which very rarely happens. I'm usually able to finish a project and archive the stuff to offsite drives before I get too deep into the next one. I could also add more Dropbox storage if I needed too.
One thing to keep in mind, is that if you are syncing large amounts of data, the initial sync will take many days.