I have my HR20 hooked up to my TV (Hdmi) and my slingbox pro (composite) and the picture transfers to both devices without any setup changes to the HR20.  I have the HR20 setup to output 1080i and the 480i signal is output through the composite cable, even the HD 16x9 channels and recordings.  (I also have the S-video output from the HR20 to one of the other inputs to the TV and it also is visible even with the HR20 set to 1080i).

This was different than the HR10 I used to have.  In order to switch between HDTV and slingbox viewing on that unit, I had to change the output resolution between 480i and 1080i.  The only thing I have to change with the HR20 is the screen format (between pillar box and stretch) when watching SD programming/recordings.  That remote function is included with the SlingRemote for the slingplayer PC and slingplayer mobile.

His comment is relevant.  The key is he's using HDMI and component at the same time (and SVideo for that matter).

Slinging Tivo Series 2, Tivo HD, and TW cable from a Pro. Slinging Tivo HD from a Solo, Slinging Humax DRT800, and TW cable from a Classic to Sprint Mogul, AT&T SX66, N62, 8525, and Dell Inspiron 8600 with Sprint EVDO over Windstream DSL.

I have an SB AV working with my HR20-100. The TV uses HDMI and the Sling S-Video. HD programs display on my Notebook w/o problem. You do need change the Notebook View format to 16:9. No Setup changes are required to the HR20-100. Leave it alone or you will mess up your regular viewing.