So the Sony isn't HD but will be heavily used, right? What's the best video connection that the Sony offers? I ask because I've got the beginnings of an idea, but it may take a bit of testing and (at least in its current form) it may involve compromises you don't like. The s-video output from your HD cable box
should pass everything except the HD stations (in the case of my Time Warner service, that'd be stations 1 thru 799). My thought here is that you could run s-video from the HD cable box to the TV for regular viewing and run component video from the HD box and Xbox to the 990, with the 990's component output going to the projector for HD viewing. If your Sony supports component video input, you'd be sacrificing that (although how much of a video quality hit you take for that is something that depends on the exact TV and the sources in question), but otherwise it should be a fairly effective solution.
If the s-video route works for you and you want to get really fancy with it, try this on for size. Connect the s-video, component video, and digital audio outputs of your HD cable box to the 990. The key here is to connect the s-video and component cables to different video inputs (we'll say that the component goes to video1, the s-video goes to video2, and the digital audio goes to coaxial1 for this example). Label Video1 as "HD Cable" with component-vid1 and coaxial1 assigned to it. Label video2 "SD Cable" with s-video and coaxial1 assigned to it. Then run the s-video monitor output to the Sony and the component monitor output to the Sanyo. This way, you can simply select Video2 to watch SD cable on the Sony (with all menus and channel guide, plus the 990's menu) or Video1 to watch HD cable on the projector. You could even use trigger 2 to lower the screen (with the trigger set to activate when Video1 is active). Between the six video inputs (including the front, which can have a rear digital audio input and rear component video input assigned to it), you could have several sources configured this way. The only component that could potentially resist this approach is the Xbox, although if you were to use the s-video adapter then you could get a signal to both sources (the 990 would transcode the s-video up to component for the Sanyo).
Nice sig, by the way.