Well, to be honest, I'm not sure the screen is drifting, but as Hank Hill might say, "There's something not quite right about that screen." : )
The 720 I'm currently using is more stable than my 700 was, but even so, every once in a while I'll tap squarely in the middle of a menu option with my stylus or fingernail, and the menu option above (it's always the one above, never below!) will select instead. I will put up with this behavior for a while, but over time it will appear to become more frequent (perhaps just via perception bias, though...I don't keep a log of the events to analyze it objectively), and eventually it will bug me enough that I'll recalibrate.
I actually ran into that problem several times last night, so I recalibrated again (probably the 4th or 5th time I've done it since getting the unit in November), and the results made me wonder if there was a hardware issue. I carefully targeted the center of each crosshair with my stylus, but despite the care I took, the calibration routine took me "around the screen" 3 times before it was happy and gave me the option to keep or reject the new calibration. Out of all the time I had my 700 and the several times I've recalibrated the 720, that's the first time I've ever seen the tool force me to go around the screen more than once.
I have tried to use the built-in note-taking tool that lets you draw free-form on the screen, just to see how well the lines follow my stylus, but the window available for drawing does not include the edges of the screen were the calibration deviation seems to be the worst, so I really can't get any direct feedback to see how good or bad it's tracking along the edges.
I used to have a similar issue with a Palm V PDA I had back in the day. Kinda figured it was just a characteristic issue with this technology, but if that's not true, then I'd really like to find a solution.