We have to watch where the back, Windows and Search buttons are. Because they are no hard buttons, just swiping them with the side of your thumb will press them, taking you away from your game. The closer they are, the higher chance you will hit them. Some games from the market place I always hit them when I play.
What handset are you using? Samsung fixed that problem ages ago on their devices.
With the new touchscreen driver from Samsung, if you drag off the bottom edge of the screen, the phone automatically disables the capacitive buttons. You can't hit them unless you fully lift all your fingers off the touchscreen, then press a capacitive button. This totally eliminated accidental-exits in games for me.
In any case, I did take the capacitive buttons into account when designing the portrait configuration. The Select/Start buttons are positioned to land above and between the capacitive buttons on the bottom of the phone (rather than directly above a hardware button), making it very difficult to accidentally set off a hardware button.
Landscape is just difficult all-around, due to how little extra space there is around the game screen. If I'm honest, if portrait were available, that's all I'd ever use because the controls lay out so well (and it looks like a gameboy, hehe).