I used to own a Pinnacle PCTV Togo and Hava Platinum. Both boxes supported an unecrypted RTSP feed for local multicasting.
Sling on the other hand uses SlingStream 2.0. It is a proprietary encrypted VC-1 or H.264 stream depending on the attached client. The other caveat is that the Slingbox will dynamicaly optimize the output resolution based on the statistics of the received and dropped packets.
But, I'm with you. I'd like to see Slingmedia add multicasting to their lineup against their competitors like Pinnacle and Monsoon Multimedia.