Hot swappable drive cameras already exist. I don't have the model numbers right in front of me, but they exist in various forms. The technology is still a little flaky, but the future is clear and it includes hard drive media for digital video cameras (followed by RAM-based capture of some sort). Hard drive video capture starts in the prosumer line and costs around $2000 to get started. Add another $1000 or so for a separate portable unit with swappable slots. They're great for what they do. Go back to the studio and start editing right away, no tape to capture. I've been thinking of getting one for a while now, but can't quite justify the expense at this point.
Another cool feature of one particular model (a Sony one I think) is that it is essentially always recording. It buffers a continuous thirty seconds of recording, which is great in case you miss a critical shot. As long as the lens cap is off, you're sure to capture. Audio keyboards have done that for years, buffering audio/MIDI recording "always on" in case you happen to play that one great performance when the sequencer was not running.