Gamerz Theatre has a great little article on putting live-filmed pilots in Machinima aircraft.
This is an excellent idea - particularly for close-ups, which they only just touch on. You could extend this to Eve Online Machinima, various racing games, and even Medieval Total War - a good way to combine the strengths of Machinima with the strengths of DV.
If you’re doing this, make SURE you get a clean green-screen (easy enough to do with some green fabric and some bright halogen work lights, which cost about $15 each), and pay attention to matching your lighting. Lighting will make or break your composite.
I’m really interested to see more of this work.
UPDATE - there must be something in the air today. WoWInsider is showcasing this marvellous video mixing WoW characters with real-life footage, rather than the other way around - very sweet, and I highly recommend it.
And a few days ago, Free Pixel was talking about using Augmented Reality to film real people next to Second Life characters in real-time - very cool indeed.