Those are some good ideas!
Stop watch and instant reply could be used in combination if there are times that are close calls [times that are within the same second]
I was thinking about the physically triggered switch but that could have limitations if the rider does not trigger it for whatever reason (pedal wheelies at the finish, does not apply enough pressure to trigger it). Also will need to be built to withstand extreme use.
Another idea could be using laser triggered timers. The circuit should be pretty straightforward (idea:
EVENT TIMER PRO). The drawback is the laser's operation in daylight or lack thereof.
Best case is if there is a trigger unit at the start and at the finish. Times are recorded and the difference is the rider's time, all done seamlessly in Excel or similar program.
Then again depending on the size of the race there will always be the need to have human supervision in the case of passed riders / dropped out riders which will inevitably introduce human induced errors.
Perhaps the basic stopwatch in combination with instant replay is a worthy contender given the simplicity and accuracy when needed.
Btw, sucks to lose by msecs :-P