If I input 1080p thru the 990 do I get 1080p out?

I assume you are talking about the DVI input, and while I haven't tried it (my HDTV doesn't support 1080p) my understanding of the 990's DVI switching would suggest that it would certainly handle 1080p. The component video switching is designed for 720p and 1080i, and will not pass 1080p - but since the only 1080p outputs around at this point are HDMI/DVI I'm guessing that's not what you're concerned with.

This raises a question I've wondered about: With a very high-quality DVD player output or a cable box with a good HD signal input, how much of a visible difference is there, really, between video via component cables and via HDMI/DVI? This is with material actually available to the consumer, not lab stuff.

With my cable box, the differences between component video and DVI are effectively nonexistent, but then my HD display is an analog device (direct view CRT). If you have a digital display (LCD, DLP, LCoS), you would be forcing the video to convert from digital to analog and back to digital by using component video.