Personally I think that Powell could hurt McCain more than help him:
He was viewed as a more moderate than most of the other members of the Bush admin., where as McCain will need a more conservative running mate to
shore up the conservative vote.
Remember that he was a key figure in delivering the erroneous information on Iraqi WMD programs to congress(although at the behest of Dick cheney),
Dems will jump on that in a heartbeat.
And he may not be another old white guy, but he is still an old guy, and I think he needs someone younger in the white house. Remember its not always
about politics, the V.P. does need to be able to carry on the rest of the term as well should the president die
Colin Powell does however have upside to him: His popularity rating never wavered much through the tumultuous first term, and being from Harlem he
could garner alot of working class voters, but he just hasn't shown much political interest in the four years since hes been gone from the white