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.
Here is a link to a full tutorial about running MySQL on EC2 with EBS: http://developer.amazonwebservices.com/connect/entry.jspa?ex...
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.