Quick pointer to a great bike hack over at BikeCommuters.com. The picture pretty much says it all, but basically you cobble together a spare seatpost, saddle, seapost-mounted rack, and some panniers.
Bingo, you've got a hot-swappable cargo module. Keep your bike free of cargo cruft until you need it, then drop in this modular cargo rig.
Extra bike hack points for the seatpost collar mounted low on the post as a slide stop!