Also would send a message to the Democrats she is more about power than principals if she abandoned her Democratic party.
If Clinton and McCain want it, they'll figure out some way to make it happen. I don't know if it's even a possibility. It may just be a great soap opera twist that some bloggers are putting out there for fun. However, if it wasn't for the fact that Hillary standing in front of the Republican convention waving would likely cause bigger riots than Hillary in Denver would cause, I suspect he would seriously consider it.
The way the constitution is written, there is no requirement that the VP be of the same party as the president. The only requirement is that the VP get more electoral votes than other VP candidates.
The whole “party” thing is outside the constitution, it's just a structure we have accepted and are used to.
Someone could run as VP, as an independent, and win, and serve as VP no matter who the president is.