Congratulations, all present and future PSP owners: You get a free defective square button, courtesy of Sony and their hardware design team. Apparently, because of space limitations due to the large screen, the square button sensor had to be placed at an angle below the button itself, instead of directly below it. As a result, the square button is universally less responsive than the other shape buttons. Oops.
What's funny, though, is that Sony isn't apologetic at all. Their stance on the issue bacisally equates to, "We designed it this way, deal with it." I don't know about you, but if I were playing Gran Turismo on the PSP, I sure as hell wouldn't want my brake button to be anything less than responsive.