This update fixes two bugs in CS5 (no need to update if you have CS3/CS4 - these were in CS5 only):
- the undo bug (also caused problems in 4-view mode)
- the aliasing / moire / cross rendering artifact
Get it from Red Giant: http://www.redgiantsoftware.com/blog/2011/02/14/showing-great-form-1-1-1-update-available-now/