"May 5, 2010" is probably wrong. Amazon has it set to "Dec, 2009" and Ebgames has it set to "2, 2010"
Yamauchi stated that they could release the game any time they wanted. And the latest by Yamauchi on GT5.
"Gran Turismo will come out on the PSP the 1st of october first. The only thing I want to tell you today, is that GT5 will be released shortly after this date."
NewEgg saying that GT5 will be released on May 5, 2010 it's a big statement and without a fact.