Also quite interested in this issue. I have the white wifi model, SGP611 I think, and I was not able to use it as reliably for school as I had intended because the screen would stop responding properly to inputs while I was writing on it.
Overall, it works pretty well, but when I need to handwrite on the screen, it frequently fails to recognize inputs to particular spots. The marks will sometimes appear closeby or not at all. With persistence, you can color in the spots that started out blocked, but there are usually many such spots at once. Although, again, none of them seem to be permanent. The screen sometimes works completely as it should.
I'm still really curious about this even though I have long since moved on to the awesome Z4 (I kept the Z3TC though)
