Corel has released the long-awaited Hotfix for Painter 12 Mac OS 10.7 Lion. Read about it here, then download:
Be sure to back up any custom items you have, brushes, color sets, paper textures, palettes, etc. before doing anything.
Note: Although this isn't stated on the page, I strongly suggest that after you install the Hotfix, you reboot your computer, then on the first launch of Painter 12, hold down your shift key, and choose "All Workspaces" from the dialog.