EBS does deltas, but there are a few caveats. The most important being that you need to be using EC2. For many, $72/mo plus bandwidth might be a bit much for what they're doing if it can work on a 512MB Xen instance for under $40 with a few hundred gigs of transfer included.

Beyond that, drive snapshots aren't the easiest things to do. I know that Right Scale tells their customers to freeze the drive so that no changes can occur until the backup is complete. With S3 performance around 20MBytes/sec, to backup 1GB would take around a minute. That's not bad and only doing deltas it's unlikely you're going to have a huge amount to backup at any given time, but it isn't exactly good either. With file-level backup, you can do a mysqldump and then just back up that file. Eh, maybe I'm just preferring the devil I know in this situation.

It's a little more complex to set up (doing file-level backups), but if you're going the volume route, you need to make sure you don't leave the drive in an inconsistent state.

All that said, EBS is awesome. If it fits what you're looking for, then go for it!