I know of two options:
The Porsche RTS (a fancy skeletal roof rack), which I've seen on eBay for around $400.
The other is the Willwood system, which gives you a rear hitch:
http://www.willwoodsystem.com/
Has anybody here used the Willwood system? It looks decent enough, but I'm not totally sold on the durability aspect. Specifically the single hidden receiver hitch behind the rear plate setup.
Then again, with the RTS, you have a muddy bike sitting over your head. I'm not sure which I'd prefer.