Is there a keyboard shortcut for playing a song next in iTunes DJ?
You didn't mention what keyboard, but on the Apple keyboard:

F7 - restart current song

F8 - pause/play

F9 - skip forward

Note that you need to have the box unchecked in OSX that says use function keys as normal. In the event you use your function keys for something, I've also successfully created trigger hotkey combos in Quicksilver. You can also probably just remap them in keyboard preferences.