To amplify on Dave's comments a bit. We are hearing from manufacturers of video scalers and HDMI switchers that more and more manufacturers are not following the current HDMI specification completely and are not providing a 480i output resolution. Also many of these manufacturers are suggesting that HDMI switchers include scalers to deal with the EDID and HDCP handshakes between multiple devices and the display.
If your 3x1 HDMI switcher includes a scaler then you are probably safe. If it does not and is simply a switcher passing through input x to output then maybe. The fact is that even if HD-DVD comes to market and has tons of software support it does not effect what will come out of you cable or satellite box. Nor will you likely junk you current DVD player and DVD collection so it is quite likely that you will want multiple resolutions transmitted to your display. I could easily imagine wishing to send 480i component to a scaler to be upconverted while I might wish to pass through native higher resolutions. Then again, I think most plasmas look better when sent 720P so you might wish to downconvert 1080i or lord forbid 1080P to 720P via the built in scaler.
Picture quality and flexibility aside the real issue is the handshake and the scaler makes it more reliable between device and display.