There is probably a potential lawsuit against mybitcoin.com for negligence.
Numerous users have complained about the theft of bitcoins from mybitcoin.com
http://forum.bitcoin.org/index.php?topic=20427.0
If they were aware of the potential security breach and did not take adequate measure to protect their members they could be liable.
They would be held to a reasonable standard.
It seems Tradehill and Mt. Gox halted trading until passwords were reset to protect their members accounts.
That is the standard they should be held to.
I am not clear if mybitcoin went that far.
They have this message on their site:
IMPORTANT NOTICE - PLEASE READ!
MTGOX was recently hacked. Their password list was stolen and leaked to the internet. If you have a MTGOX account and the password is the same as your MyBitcoin account you MUST change your password immediately or risk having your account compromised. You have been warned.
If that is all they did it would not be sufficient compared to what reasonable competitors did, and was not sufficient as many users lost their coins.