I said it on another thread, but there is NO way McCain is becoming a Democrat or joining Kerry on the ticket. McCain is the co-chair of the Arizona committee to re-elect Bush and he is a stong (albeit independent) Republican. I don't think he is going to run for the presidency again, unless there is a groundswell of grassroots support to do so. Many of the GOP heavies do not care for his independent nature.
As far as Ustwo's comment about the "ultra-liberal" Kerry White House, I think he is off the mark. I think President Kerry will put together a "center - left" administration and frankly someone like McCain would fit in well, much like former Secretary of Defense William Cohen(a Republican) did in the Clinton Administration. But I guess we will find out next January when Kerry takes office.