I noticed this problem with my Toshiba as well. At first I thought as you did that the calibration was in some way nonlinear. But I later realized (on my unit, at least) it had more to do with the fact that when writing near the edges (or especially moving scroll bars near edges) I was holding the pen at a different angle with respect to the screen. If I hold the pen with the same strange angle near the center it also is inaccurate there.
I was able to (mostly) fix the problem by orienting the pen normal to (straight out of) the writing surface at the edges. It also helped somewhat more for me if I held the pen straight out while doing the calibration. That seems to keep it as accurate as possible over the whole screen.
Since I became aware that the change in angle is what was causing the innacuracy near the edge, working around it really hasn't been inconvenient (i.e. it doesn't feel awkward). Would be nice if pen angle didn't matter so much though I guess :)
Hope that helps,