Dunno. He apparently approached the Kerry Kamp in '04 about a vp position, but when Kerry contacted him about actually going there, McCain made a huge deal about how different they are and how McCain could never sell out his principles like that. IMO, fat Teddy probably told him not to, because he could then make even more troouble to his nemesis, President Bush, as a Republican.
Now...well, who knows. McCain would probably lock the election for the slimy creature. Look how many people keep voting for him in the primaries. UGH. On the OTHER hand, McCain also has a very good chance of beating the slimy creature...so he may well refuse again.