Re: the color pixel detection: notice those gray "splotches" (I can't think of a better word for them between the quadrants in the squares? Those are intended to improve accuracy by not registering movement into them. This means doing a getpixel under the pen location, which means, programming-wise, that I'll have to convert the server-side pixmap to a client-side one. But I didn't know GTK+ at all until last week, so I'll have to do a bit of research.
As far as riding the bezel goes, I can't reproduce it myself (but then again, I'm running in landscape mode, so I have only one bezel that's really an issue). I suspect some sort of rounding error, and I have some ideas on how I could fix it, but I'd need you to beta test packages I'd send over to you, because I don't want to cut releases with changes I'm not that sure about...