Downloadable version of the game only lets you play with YOUR PSN ID & logged in.
TripleTags.com's SectionZ writes:
"Well I cannot really say I am surprised by this. And the security on the downloadable version of Gran Turismo 5 Prologue is a vast improvement on Warhawk's, which does not let you even play on your second PS3 within 24 hours of playing it on the first. However, I am kind of saddened for my broke ass siblings with whom I share my PSN downloads with, since GT5P only lets you play logged into the account that you bought it with."