Tampering with that mail is a federal offense, of course. On election day you use secure http to sign in from anywhere with your name, address and password. A dot-matrix printer makes a good emergency backup medium. This system also needs a dirt-simple GUI for voters connecting from home or work. At the voting booth it can be even simpler, using touch-screens.
Security is, of course, always a problem.
Also, net admins overseeing the effort need to have enough access to track and lock out attackers, but obviously they can't have access to change the election results. Lock them in a room for the day with a hundred video cameras tracking everything they do, like the officers on missile-launch duty. Many net admins will find this a relaxed and enjoyable work environment compared to their current jobs. My hunch is that they can be solved. Given that Florida's election is being decided by a 400-vote difference, with 19,000 botched votes thrown out, I'd say the impossibility of clicking on two presidential choices at the same time makes this system a huge win.