Not as hard to implement as some of the pipe dreams out there. Of course, it does require a degree of tech savvy on the part of users - and more importantly, enforcing it's use, to avoid laziness bypassing.
Then your challenge becomes certificate transport - you'll need a way to carry around your cert, or somehow get hold of it when you need it, which is easier said than done. The real advantage of passwords is their portability. Biometrics have a similar advantage, but as already noted - are a bit harder to revoke/change.
Life would be so much easier if we could just look at the source code. -- Dave Olson