Last September, Microsoft bought the software bits of Creature House, suspended their sales, and are now giving away an extremely powerful graphics system, Expression, for Mac only, for free. Simon Cozens: Expression
I downloaded it and scribbled with it briefly. It's definitely a Carbon app, but it feels zippy under OS X, supports Photoshop plug-ins, and will likely be fun for artists to noodle about with. Even with just a mouse you can do some neat sketching (perhaps I'll post my "rooster head" at some later time).