If you're really hardcore about Guitar Hero and Rock Band, and are always on the go, you might like the NEXiLUX (Meximelt?) Mini Guitar. This is a retractable, packable stick guitar that works on the PS2, PS3, and Wii for both of the above listed guitar games.
The Mini Guitar should be available in the next two weeks. No price or exact release date was given, though we do know that it is offered in both black and white. The video shows how the whoel thing is put together. You can cram a Wii Remote in the back of the unit, and the wireless adapter/dongle is also shown off.
This looks to be quite a bit more handy than toting a full-sized Rock Band or Guitar Hero guitar on-the-go. Of course, those serious about the game would have to see how well the controls function. Is this something you'd consider using?