What's the best routine for a quick lunchtime workout?

Matt Hart, personal trainer, suggests these excersises to get you in and out of the gym fast:
Day 1 - chest and triceps:
The bench press works the chest and tricep muscles and a tricep push-down using the high pulley will isolate the triceps to finish them off.
Day 2 - back and biceps:
The chin-up hits both the lats and biceps, as well as working all the core muscles (abdominals, obliques and lower back) and a bicep curl with dumb-bells or low cable is the perfect bicep isolator.
Day 3 - shoulders and legs:
A military press using the Smith machine will stress both the deltoids (shoulder) and the triceps again (use dumb-bells if you have no Smith machine). Finish off with a free-standing squat or use the Smith machine and you'll work your glutes, hamstrings and quads in one go. Modify this exercise slightly so that you push up onto your tiptoes at the end of the movement to work your calves.