Somehow I failed to mention the Freescale i.MX (dragonball) series. Something there might be your best approach. Some of them have video decoders and LCD controllers built in. You’d probably have to start by coughing up the $$ for a development board. It appears there are a few third parties.
Freescale is apparently in some sort of packaging patent dispute that may affect availablity of the i.MX chips in the US. I don’t know the details.
I recently looked at the Propeller datasheet myself. Intriguing concept (especially if you’re wrestling with the “to nest or not to nest” issue as your ISR’s grow like weeds). And the built-in video generators look very handy. Nothing there to help you with decoding though, as far as I can tell.
I think you’re right about the LPC2378. Great (or at least adequate) for a nice GUI. Not fast enough for live video.
I also failed to mention Conexant and Empia, who seem to be a couple of the major suppliers for consumer grade video interface chipsets (like the Hauppauge HVR-950 USB tuner that I own). But I have no idea where to get technical documents on their chipsets. It is a mystery to me how the video4linux people got enough information to create driver software for so many of their devices.