Accompanying the Unreal Tournament 2004 preview on GameSpy.com based on seeing Epic's upcoming shooter sequel at the CPL event, is a Q&A with Cliff Bleszinski to interrogate the Epic designer on the topic. Here's a bit on the oft-discussed plan to add vehicles to the next iteration of the UT franchise:

If I listed every last vehicle, then there'd be nothing to look forward to! We just got in the updated three-person Hellbender. (It needed a makeover, the first design was too similar to another huge franchise's buggy that everyone knows and loves.) It now looks like a Humvee on crack/steroid cocktail with both passengers manning their own weaponry.

Vehicles are a lot of fun, but they can change a game dynamic significantly, and it requires a bit of finessing to keep things fun and balanced. We don't want the gamer to feel useless if he doesn't have a tank, so balancing each vehicle's strengths and weakness is an ongoing challenge (especially when you insert vehicles into a game dynamic that already has 12 or so established and well-balanced weapons).