http://archives.cnn.com/2000/LAW/08/...clinton.08.01/
written by vice dean and law professor at columbia univ. leading up to the 2000 elections concerning the gore ticket and whether it could include bill clinton. it is dated, but addresses the issues discussed here.
basically says someone like clinton, or in the future W, could run as veep, and then serve as pres if needed. obviously, there would be challenges such a situation. but this prof says it could legally happen, however unlikely.