Ask Slashdot: Low Cost IP-based Traffic Shaping?

Deuteron asks: "Hi! I work for an ISP and we're about to deploy wireless net access and need a way to limit people to the bandwidth they pay for. We're planning on starting out with offering 128k, 256k, and 1M links. The wireless hardware itself (Breezecom if you're interested) will handle the 1M part for us. The tricky part is the lower speeds. I've done some extensive checking and haven't found any IP or MAC address based shapers as of yet. Can anyone point me to some free or extremely low cost solutions? Any leads would be greatly appreciated!"