Funny you should mention this. I emailed MyBitcoin yesterday. I asked those exact same questions. Great minds think alike, I guess.
I was told that they do hourly snapshots on a 90 day loop to 2 remote servers, and that the server they are on has RAID, etc.
They also told me that they back the "deposits" 100%. I wonder if they'll publish stats. It would be interesting to see their payment velocity, average account holdings, etc.
Hey FreeMoney -- can you vouch for MyBitcoin? It looks like a pretty handy service. However, I'm curious as to how well backed up the coins are. I back up my wallet in several locations after each transaction. I wonder if MyBitcoin has an intelligent backup system... what happens if I use myBitcoin and the server's HD dies?