I think a reverse DoS attack would be one that actually increases the capacity of the service. I'm not sure what scenario would allow that to happen though.
Some streaming sites (e.g. 4 on Demand, IIRC) have a peer-to-peer element; you could reverse-DDOS those by running a lot of computers "seeding" (perhaps by leaving them on the page but at the end of the video) distributed all over the globe.
