Topeak makes a rack you can mount to your seatpost and uses a quick release so it's easy to take on and off. They have a rail system on the rack to go with their bags. It's not a HUGE bag, but large enough to hold a lunch, tools, deoderant, wash cloth, other small things. Not big enough for me to fit all my clothes and shoes in though. It's really easy to put on and off the bag, so you can take it in to work. The actual rack has a 20 pound weight limit. It could work if you are not planning on bringing all your clothes to work.

http://www.topeak.com/2006/products/bags/bagsmain.php
http://www.topeak.com/2006/products/...txbeamrack.php