Have you tried holding "Command R" while booting? It will take awhile.
What happened to your recovery partition?
If this is a new 2012 iMac, I don't think the App Store Mountain Lion download will work on it, the new iMacs come with a slightly newer version of 10.8.2 which hasn't been released to the public yet.
If it is still in running condition, you could encrypt your disk with FireVault and then let Apple sort it all out when they get it back.