Syncing is not Backup
Dave Greenbaum points out, quite well, that syncing is not the same as backing up.
http://theappleblog.com/2010/03/26/syncing-does-not-a-backup-make/
It is a really great post, which notes the distinction between making something available in multiple places and actually backing up. MobileMe, DropBox, and Sugarsync, for example, are not online backup services, they are sync services, and while they offer some potential to use file versioning to retrieve files, there is no guarantee! Of course, Mozy doesn’t guarantee anything either. But some folks do, like The Backup Plan 
iPad backup considerations
My guess is that Apple will sell 1 or 2 iPads this weekend. If they do, then there will be quite a few folks out there taking pictures loading pictures, storing music, books, files, apps, etc. So, remember, this is not an item that is approved for end-user service. So, if your device dies, or the screen cracks, or the battery dies, then you may end up sending the iPad in to Apple for warranty “repair” – depending on your service options. Apple has already stated that for battery issues, they will be sending out REPLACEMENT iPads, and it seems likely that the same may be true for most depot service orders for the iPad.
SO??? So, make sure you sync it with iTunes often. That is the only current backup method for the iPad, and doing so will make you feel much better if or when you do have to swap it out for a replacement.