OSX Lion Disk Mounter
The launchd daemons and agents should be mounting these for you...


com.apple.diskarbitrationd
com.apple.diskmanagementd
com.apple.DiskArbitrationAgent


... so unless you were editing these or your filesystem had the files go corrupt it might be faster to install Lion onto an external drive and test to see if a clean install remedies the issue. 

If so, you could migrate your data to that external drive and make sure it's not a configuration issue. 

If not, you could then start down a hardware isolation or repair path.