If it went away when you restarted then I wouldn't be too worried, keep an eye on the issue and if it crops up again then maybe consider an appointment, but if its just a one off and not linked to other issues then you should be fine!
Not really. A bad videocard could generate a corrupted texture which is then used somewhere else on the system - thus allowing him to take a screenshot of it.
But yeah... If it crops again: genius appointment!
Well, it's where the menu bar (is that even what it's called?) should be so I think it's rather safe to assume that it's a messed up version of that. Might I even postulate it might possibly be a graphics rendering fail due to poor communication with the gpu/thencpu/thenscreen or a failure with the graphics card itself I don't know (I highly doubt the latter, however) but a clean install of Snow/Leopard should fix this.
I went to edit punctuation in and I saw the above post hinted the same thing I was thinking might be possible. Glad to know I'm not a f*cking idiot :R