My low leve flying experience is more with fixed wing than rotary wing, but some of the same ideas apply. As has been mentioned, I'd go with a wide angle to moderate telephoto lens as small choppers do tend to have a lot of vibration and it all gets magnified through the lens. IS would be plus. Since you're not going to have a 360 deg clear view (and what the pilot can do for you depends in part on what air traffic control will allow him to do), you are going to end up shooting through plastic at least part of the time. You'll have a four-way safety harness if you're sitting up front; sometimes you can lean a bit into it and use the harness for a bit of stability.
One more thing. In my experience (and depending on the pilot), if s/he asks if you've ever been in a helicopter before, always say yes. On a first flight they are sometimes tempted to show you what a helicopter can really do, and you may not be able to keep your lunch.
Then there was the guy who was in the copilot seat of a Twin Otter, side window down, taking shots with his Polaroid camera. He couldn't figure out why the camera wasn't working, until he realized that at 150 knots those automatically ejected, self-processing films were being sucked out of the camera and blown away in the prop wash...