I think the only way The Prez would do this is if Cheney had a heart attack or something equally serious and asked to be removed from the ticket. Otherwise, save 2008 for a POWELL / MCCAIN ticket or a McCAIN / GUILIANI ticket. (It can't be Powell and Rudy; they are both from NY.) Cheney will not be the standard bearer in 2008.
I understand there are health issues that would likely preclude McCain from consideration as a Presidential or VP candidate.