...if you really need to keep hopping curbs, and like most bike messengers you only use a couple of gears ( I guess that depends on the city you work in), you might want to consider going to a wheel/ hub combination that frees you from rim braking.

You can build a pretty bomb proof 700c wheel using a box section rim like Alex Adventurer or Sun CR18, and couple it to a SRAM two speed Automatix hub with a coaster brake. It's not quite as good as having brakes front and rear, but you guys just go around stuff anyway, right ? the 700c rim ought to give you room for a little fatter tyre in the frame, too.

You have to fool around a little bit with figuring the gearing that's ideal for you, but I have this setup on a track frame here, and it seems to work OK with just the coaster brake....and you can do way cool skids with it. It's not real practical if you need some gearing range, but it's better than single speed, and if the wheels go a little off it's not a big deal.