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!
This is not totally accurate. EBS snapshots are basically instantaneous, its just the copy to S3 that takes time, but Amazon performs this in the background. We use XFS on our EBS volumes (running MySQL 5 innodb) and then have a little perl script (http://ec2-snapshot-xfs-mysql.notlong.com/) that does FLUSH TABLES WITH READ LOCK -> xfs_freeze -> snapshot -> xfs_thaw -> UNLOCK TABLES. The whole process takes a fraction of a second, and it also logs where in the binlog the snapshot was made (handy since we create new slaves based off snapshots and reduces how much data we shuttle around).
We snapshot a slave every 10 minutes and the master once a night (just in case something totally weird happens to the slave and the sync isn't right). This is a multi-gig DB and we've had no problems.
I wanted to also point out that a live slave is NOT a backup scheme. If someone hacks your database and runs DROP ALL FROM PRODUCTION_DATABASE you've now got a perfect copy of nothing.
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!