I don't think there's ever been any confirmation that it's analogue only, but by that same token don't you think if there was digital out, we'd have heard about it? Also wouldn't they need a digital out of sufficient BW to support HDMI? If they have that ability, why haven't they provided optional HDMI cables from day 1? Seems a nice consumable you could make some good profits on to me. The market might be pretty tiny, but the development cost, if just a wire, should be minimal.
Still, with 30 pins on the connector, the inclusion of digital outs could be included. Or the connector was selected to provide forward compatibility if MS ever decided to produce an XB360 with digital out on the same connector.