There is no substitute for a cold offsite backup.
Any backup that's always online and talking to live servers, especially one in the same data center, runs the risk of either being compromised by intruders or failure due to some cause that kills the original (fire, flood, etc). For these two reasons, you probably want a close to real time live backup, in case you do something boneheaded by mistake, and a less frequent cold offsite backup. The beauty of cold offsite backups is that they are as isolated as possible from any conceivable scenario (save a nefarious individual out to destroy all your data at any cost), and while you may lose a few days / weeks of data, it's better than losing it all.
As far as backing up bad data, any backup system can silently backup corrupt data, that's what regular tests are for.
If you can get a better rate on storage than EBS from another host (not hard) you can setup that host as a MySQL slave and create your own LVM disk for MySQL allowing you to perform LVM snapshots regularly. Make sure, no matter which snapshotting mechanism you use, you make sure to FLUSH your tables and read lock them so as to maintain data integrity. See http://lists.mysql.com/replication/1741 for some more info. If you're using a read only slave you can probably just issue a stop slave and flush, though the read lock won't hurt.
Alternatively, you can just stop your read slave completely, shutting down the SQL server, then use rdiff-backup, which is an incremental backup, only backing up changes, to copy your MySQL files as well.
The real answer, however, is you probably don't need all this. You can probably get away with doin a mysqldump automatically every once in a while, gziping it, and uploading it so S3, downloading copies every so often to your home computer for backup.
