On my computer, I can hear the sounds, just as if the game was running fine, but it fails to display anything (ie. I'm seeing Mail.app, fullscreen). It doesn't matter if I play on my external display, or the builtin screen -- still happens.
The "fix" is to cmd+tab into another non-fullscreen app (this actoin should switch desktops), then D3 runs fine.
I'm running Mac OSX 10.8.2; I'll gladly provide more details, if you need them.