External display occasionally shows just noise
I have had the same problem with Maverick (in clamshell mode) / external monitor (Samsung SyncMaster 2443) when I lock my screen and use my bluetooth mouse or keyboard to wake up the computer.

I was able to fix the problem in 2 ways:

open the lid, login and unplu, replug the external monitor (pretty annoying)
when the multicolor static shows, press CTRL+SHIFT+Eject as if to lock the computer again, then wake up the computer again and I now have the normal login screen as normal (not ideal but less disruptive solution).

I hope this helps !