When I upgraded to OS X Lion, I discovered that the way that I had previously set up iTerm2 to launch tmux would cause a kernel panic when I would close iTerm2, without fail.

I don't remember off the top of my head what the setting was, but it was working in Snow Leopard on two different machines, and the kernel panic behavior struck on both upon Lion upgrade.

At any rate, changing how I had iTerm2 launch tmux solved the issue, but creating a guaranteed kernel panic like that was very odd indeed.