I think a rear touch pad could be usable.  With a front facing one, you can somewhat rest your finger on it.  But you typically have some fingers resting on the back of the PSP.  If they can make it reasonable (ie, you can touch the thing with your middle finger and rest your other fingers elsewhere without touching the pad) I think it could be invaluable for FPS lovers.  Could also be used as a more usable mouse (compared to the current use of the joystick).
Positioning would obviously be vital.  If you typically interact with the PSP using your thumbs, and pointer fingers on the trigger, the rear pad would need to suit the middle finger well.
Though I find for some rhythm games, I tend to press the front buttons with my pointer+middle finger, rather than thumbs.

(This post was last modified: 30/11/2010 03:20 AM by ZiNgA BuRgA.)