@ Baz: Thanks for the tip about the clipping plane thread. I didn't even know what to call the problem. I selected "Zoom Extents" and the clipping plane problem disappeared, but I'm glad to have learned more about it.

When I rotate my model, parts of it disappear, as if there were a plane intersecting the model beyond which nothing gets rendered. I am using a Macbook Pro with Mac OSX 10.6.7 Does anyone have an idea how to fix this? Thanks!