With the moon filling most of the frame, I found this evening that the 20D was pretty much bang on caculating the shutter speed (given it couldn't modify the aperture). Although this was only true when the whole moon was in frame - once it started drifting out of frame, the increasing black areas started to cause incorrect exposure to be calculated and the moon would start to be overexposed.
It may be different near and at totality, and I might start bracketing shots, as well as frequent checking!
Nicole - funnily enough the small telescope, tripod and adapters are probably far less in cost than a decent long prime or zoom. I think I might have spent around $NZD700 on the telescope, tripod and required adapters to connect the camera. Not to mention you can use them without a camera!