There is a nasty bug in Photoshop CS4 regarding cursors when using a brush—the icon depicting the coordinates for the cursor and where the cursor actually affects your artwork is offset by roughly 10 to 15 pixels.
Some quality time spent with the Google turned up a few other people having the same problems with a common thread being it affects Mac users with certain graphics cards. I run Photoshop CS4 on Mac OS X10.5.7 powered by a dual processor 1.8 GHz PPC with a GeForce 6800 GT powering a single 30″ Cinema Display. I have Photoshop set to display Full Size Brush Tip+Show Crosshairs in Brush Tip and can see the offset. This bug affects any tool which uses a brush; cloning, stamping, masking, brushes, etc.
Adobe has yet to acknowledge the bug exists. In the meantime, the only solution which seems to have presented itself is to jump around Full Screen (F) modes a few times to “reset” the cursor. Lets hope there is a patch in the works already, along with the one for Version Cue.