Steve I am not a Mac user, and I can't duplicate your situation, but you have gone five hours without a reply, so I'll at least bump your thread.
All I can think of is that it is to do with the cursor display problems lots of people experienced when CS4 first came out. That was a long time ago, but do still see the occassional glitch. That was always fixed with graphics card drivers, but I know Mac do that differently to Windows.
So anyone got a clue what is going on here?