All Multiplayer data is stored offline on the server tied to your account. Therefore your Multiplayer data is safe, just might need a re-download.
Some of the files are tied to your unit and account with security features and will therefore not work on your new machine, but need to be re-created.
In case you didn't already have one, here is a guide on moving data around:
http://www.gamefront.com/how-to-backup-your-ps3-hard-drive-and-save-games/
On another note, I have a PS3 friend who's disk stopped working over a year ago. They can still use their PS3 for a lot of things via the Store and networking options.
