I actually think it is more likely that they will include a DVI to HDMI adapter in the box rather than add a new input tp the already crowded backpane. The runs around $15 at cablewholesale.com, so they probably won't cost a lot more to add in than the HD component video output cables on the GeForce 6 series cards and the Radeon X800-series. Once Windows Vista is out and the video cards are made totally compliant with the HDCP that Vista will require, you'll likely start seeing these show up in the box instead of the HD component cables. Sound will probably just be sent to the receiver through the usual coax or optical cables by the onboard audio or an add-in card.
A few years down the line, once the majority of the monitor market has switched to DVI flat panels, you might finally see the HD15 disappear and be replaced with a HDMI out. By then, a lot of the surround sound receivers should have HDMI audio input support and HDMI video switching capability, so I wouldn't be shocked if nVidia brought Soundstrom back from the grave and integrated it into the video card in order to send real time encoded 7.1 audio out through the HDMI port.
I wouldn't expect anything like that to show up on the market until at least 2007, though. It'd suck to be Creative Labs if that happened, though. Maybe they would merge with ATi if HDMI audio becomes a big thing and finally kills the market for a discrete sound card. All this is just speculation on my part, though.