Happens intermittently: When I unplug my Sony headphones, I am unable to get my 17" MBP (2.8ghz intel core 2 duo, running Lion), to make any sound. I've tried a handful of different hardware, and nothing will make a peep.
Restarting "fixes" it.
Its a bug Lion has.
This thread should provide you with more info of users suffering the same bug.
This app claims to fix it (I found it on that same thread).
I haven't had the bug for a while, but I did have it quite a bit before. I really hope Apple fixes it, by 10.7.2 perhaps.
Hope it helps!
Edit: Probably the most relevant information in the thread is that it seems to be a bug on Core Audio.