I have only seen wireless load balancing in custom solutions implemented in the wireless hardware itself.
Cisco has a (roughly $20,000) controller to do load balancing and radio control for its access points; Xirrus builds access "arrays" with 4, 8 or 16 radios that will do load balancing, and I saw a note somewhere for a 3rd party firmware app for high-end netgear routers to do wireless load balancing.