Cupholders and headphones are a must.
Well, mine's a cocktail, so it'l have plenty of cup space, but I am putting a nice little brushed stainless panel next to the P1 control panel. 105x105mm, power, reset, volume (controls cab speakers only), USB 2.0 x2, S-video out, headphones/line out. I couldn't think of anything else I/O wise (suggestions?).
The machine will also serve as a video recompressor when I get my twin tuner digital tv PVR (Topfield 5000T PVR FWIW), and will also serve music via iTunes, so I needed a fast way to move files on and off (USB 2.0).
Plus, as it will only have controls for two players, the extra two USB ports can be used for analog joysticks, or gamepads for 3-4 player action.
edit: forgot pic.