Cannondale makes excellent aluminum frames. They've done really well with embracing ovalized tubing and curved seatstays. And the Capo frame is incredibly light. Much higher quality than the langster.
Some cons - and this is probably why you don't see many messengers riding C-Dales - it has an integrated headset, which many think (including Chris King) leads to faster wear. If you're riding day in and day out you don't want your frame to be ruined because your headset ground into the headtube. Also it comes with a carbon fork which probably isn't as strong as an AL or steel fork. And the price. The frameset isn't bad, but the components are 100% entry-level, which doesn't seem right for a $700-800 bike.
So I'd recommend getting the capo frameset.. this year's model is quite the looker. And you can build it up real lightweight. I built mine up with heavy parts (deep-v wheels, front brake, stock fork) and it's 16.7lbs. With track drops and no brake it's like 15.5-16lbs.