Dual screen actually sounds like it will be useful, even if games don't support it.
Think about it, you hook up one HDMI to a HDTV and the other to a PC monitor. When watching a BluRay or DVD movie, you can have all the controls that aren't handled via the remote up on the monitor while the movie is playing on your TV. I've always hated having to bring up a UI overlaid on top of what Im watching.
If the PS3 does end up being a good media server, this will be really good for that as well. You could have your file lists and controls up on the monitor, while your video or visualizations (for music) are uninterrupted on the TV.
I wonder if you can use the HDMI with analog outs for true dual screen as well? That would help out the people that only have one HDMI display device.
While not necessarily a 'system seller', I think it does add some nice convenience for those that want to take advantage of it.