I've been having some issues with my Mega this morning. It started out fine, but then suddenly it wouldn't let me upload sketches anymore ("Resource busy"). And just now it stopped showing up in Serial Ports altogether while it loops the last sketch I uploaded. I've tried connecting it to different USB ports, holding reset buttons, etc. but it hasn't helped. All the lights still come on when it's plugged in.
Worth noting that I'm using OSX 10.8.2. It shows up as a Communication Device under system information when it's plugged in.