My Plasma has:
1) HD15/DB15 (either RGB or YPbPr, 480i-1080i)
2) 5BNC/RCA (either RGB or YPbPr, 480i-1080i)
3) S-video (NTSC 480i or PAL 576i)
4) Composite RCA (NTSC 480i or PAL 576i)
5) DVI-D (*not* HDCP; PC-compatible only)
It has "wide" and "zoom" stretch modes if you feed it a 480i or 480p signal. For 720p and 1080i, you get full 16:9 "full" only. So my concern is that if I want to use only one input for the 5100, let's say input 2, and I have the 5100 set to scale everything to 1080i (including 480i sources), then the 5100 will output a 4:3 480i video source scaled to 1080i with black bars on the left and right sides. My plasma doesn't have any zoom mode than can stretch it across the screen (thereby avoiding any burn-in worries). It doesn't sound like the 5100 has any ability to stretch or zoom a 4:3 mode when scaled to either 720p or 1080i.
I could probably switch the plasma over to the s-video input for non-HD channels and let it do the stretching but that's a major PITA. I already went through enough trouble setting up macros on my MX-500 remote