From my days working at Mapquest, I recall that we had a route optimization tool but only offered it to paying customers (i.e. not available on the free web page). Companies like FedEx and UPS pay a good deal of money for tools like this. I doubt the average user could afford the GPS that has route optimization built in!
All these Garmin series offer true route optimization. Nuvi 7x0's (since 2007), 7x5's, 8x0's, 8x5's, nuvi 5000. Some recent Magellans's also offer it as did the HP310. It can be done by TomTom and won't add much if any cost. I suspect that at some point they will.