From what I've read so far, it outputs HD signal (not sure if it's 720 or 1080, though I'd probably say the later is a safe bet) until you start recording. At which point the output goes down to 480... Apparently there's a slight pause though between pressing record and when the screen starts showing what you record (it apparently blanks out for about two seconds).
Can you view your still shots in high def this way also? It would be a nice to preview your images on a 40" HD TV.