My old computer used to reset itself periodically, and would come up on metric. Sure made me feel good, until I figured out what happened... wow, I've been holding 30 for the past 20 minutes...
On the other hand, a 'metric century' just doesn't compare to a real century, in terms of really putting the endurance to a test. 60kph on a downhill is coasting, while 60mph on a downhill is exciting.