I didn't even know you could use the psp as a rear view mirror in prologue. I will have to check this out. As for as using it for that in GT5, you should ask in the GT5 forum.
This is not actually possible in prologue. Maybe something like it was demonstrated at some expo such as E3 because there was also an f1 game that demonstrated using psps as wing mirrors at but it never made it into the final game either.
If you think I may be mistaken I would love for you to prove me wrong though....
