I'm not saying it can't be done, just that only a small number of the people buying very fast routers need this, so building this functionality inside generic routers probably isn't the best way to go.
I believe it can be done in lower cost generic routers using existing commodity silicon. For sure it is available today from multiple vendors in products costing less than US$ 5000 (street price). At 100 Mbps performance, it is available in shipping products for less than US$ 1000 (street price) today. It is available at 10 Mbps performance in really really cheap equipment for home use today for ~US$ 100. Putting it in the router is unlikely to increase the router cost and many routers will be able to do it today (e.g. tli's comment about the 2600). Ran rja@extremenetworks.com