I've seen this happen with CS5, 5.5, and 6 (on Mac OS 10.6.8 and lower) and on multiple computers. I'm using the down arrow to move down through text or I'm selecting a bunch of text at the end of a file when suddenly I'm on page 1 of the document. If I was selecting text, the highlighting continues at the top of the first page.
I haven't seen that on my installs, but did you try resetting preferences?
Yup. Done that. But this happens occasionally both at work and at home and I've noticed it in several versions of CS. Not anything major, but odd when it happens.