All brushes have been reset and are set the same. We are running CS5.5 Master Edition on a brand new 27" iMac 3.4 ghz Intel Core i7 Quad core using a Radeon AMD HD6970M.
This is a sample of how the tool is not using the entire diameter. This could be either erasing or painting a color.
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?