I found a lot of articles referring to a 'rumor' and 'speculation', but no confirmation, and from McCain's own mouth thatKerry never offered it.
This one doesn't indicate that Kerry really offered it, just mentions general speculation:
(AP) Republican Sen. John McCain allowed a glimmer of hope Wednesday for Democrats fantasizing about a bipartisan dream team to defeat President Bush — a far-flung notion the senator's staff quickly squashed.
McCain said in a television interview that he would consider the unorthodox step of running for vice president on the Democratic ticket — in the unlikely event he received such an offer from the presidential candidate.
"John Kerry is a close friend of mine. We have been friends for years," McCain said Wednesday when pressed to squelch speculation about a Kerry-McCain ticket. "Obviously I would entertain it."
Within hours, the Arizona senator's chief of staff, Mark Salter, closed the door on that idea. "Senator McCain will not be a candidate for vice president in 2004," Salter told The Associated Press, saying he spoke for the senator.
http://www.cbsnews.com/stories/2004/03/10/politics/main605167.shtml-----------------------
This shows a transcript where McCain states that Kerry never offered it to him:
SNOW: Now John Kerry, is it true that John Kerry asked you to be his vice president?
McCAIN: Uh, no. No, it was never offered.
SNOW: It was never offered. So, it may have been discussed elliptically, but never flat out request.
McCAIN: Never was an offer, no. (...)
SNOW: Right. But, so- I want to just lay to rest once and for all: never approached you, never hinted that he wanted to talk to you about being vice president. All that kind of stuff was made up.
McCAIN: Well, I cannot attest to that. All I can tell you is my conversations with him were private conversations, but he never offered it.
http://chris-bowers.mydd.com/story/2004/7/8/12341/76664