I got my Gp2x today, and I stumbled across the test mode. I went to settings, and the icon that looks like a plug on the upper left, and it brought me to a test mode. pressing L and R brought me to page 2, which was an input test. Having a platform game in mind, I was testing swiftly going left and right, and I often found myself hitting diagonal up+left instead of left. right was a little easier to get to.
Though I really wish the thumbstick could be improved. Due to the way the thumbstick is, physically, it may not be possible to just have a simple solution like disable the diagonal switches. It'd be hard to fix it in software too, I mean, if you have a game where up and down makes you climb up and down ladders, left and right makes you walk, you'd very frequently find yourself accidently climbing on ladders if you left diagonals alone.
It'd also be kinda hard for GPH to fix the hardware too, because hundreds of GP2x units have gone out already, and if everyone wanted the new hardware, we'd all have to send our GP2x's back. That's gotta be a pain for the people who have to handle that mess. :S