Having only recently upgraded to Tiger, I never got around to setting up my Wacom PenPartner tablet until yesterday -- and I found that the latest driver does not include any mapping features at all!
This makes it impossible for you to draw a circle if you have two monitors, or even one widescreen display like an Apple Cinema. Your tablet's surface is automatically mapped to all available screen area, by default. You can choose to map to only one monitor, but even that's no help if you use a display which is shaped differently than your tablet.
I had some driver install troubles, which Wacom tech support fixed right up (always great response from their team), and I mentioned the lack of mapping once I got it installed correctly. Chris L. at Wacom kindly gave me a link to an older driver that still has the tablet mapping feature. I removed the current driver, installed the old one, and all works fine now. I can adjust mapping, draw circles just fine on either monitor, and use it like it's meant to be used: like a pen.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060214141025517