"Republicans for Obama" and the various media reports that Obama will appeal to many GOPers aside, McCain has greater support among Democrats than Obama does among Republicans. A Pew survey taken earlier this month shows 10% of registered Democrats supporting McCain, while 7% of registered Republicans support Obama.
Those figures are nearly identical to what occured in the '04 Presidential election, when 11% of Democrats voted for Bush and 6% of Republicans voted for Kerry, and also in '00, when 11% of Democrats voted for Bush and 8% of Republicans voted for Gore.
Obama is no more attractive to Republicans than other Democratic Presidential candidates have been over the last decade and McCain is no less attractive to Democrats than other Republican candidates have been over the same period of time. Those who insinuate otherwise are being disingenuous.