I don't normally make it a habit of lugging my Xbox 360 around to other people's houses, but should the desire ever arise (or if you have a car to do the lugging for you), MojoPlay's MP-920XB is at your service. It's a 9.2-inch traveling display that connects to your 360 and lets you play games when someone else is hogging up the TV. The screen has an 800x480 resolution along with built-in speakers and dual headphone jacks. It's out today for $170 with PS3 and Wii equivalents slated for next year, though personally I wouldn't wanna play any console games on a display smaller than 26-inches.
Actually, I have been looking for something like this. I've travelled with my 360 to competitions around the country and found that most hotels don't give you the opportunity to hook up the unit to the tv in the room. Most hotels have their own proprietary AV system with the extra inputs locked up or not avilable.