I've also been researching this issue and auditioning various systems. At first I was very attracted by HDMI-enabled systems due to the simplicy of the cabling. However, it has been my experience that the HDMI is really only used to transmit digital video signal from the receiver to the display. In typical HTIB setups, the receiver also serves the purposes of DVD player, tuner, and amplifier. Audio signal through HDMI to the display is usually moot as the meat of your sound should be coming from the speakers, which are connected via standard (or in some cases proprietary) audio cables.
The whole situation had me puzzled because clearly not a whole lot is being done with this integrated cable format. You could imagine, for example, connecting an external disc player (say, one of the emerging HD-format players such as BD or HD-DVD) to your HDMI-enabled HTIB receiver where the HD digital audio/video signals are input into the HTIB. The receiver then parses the video siganl through HDMI out to the display and the audio is dispersed to the speakers. HOWEVER, it has been my experience that current HDMI HTIB systems only have one HDMI out and no HDMI inputs. Sooo, the utility of HDMI seems to be rather limited.
Long story short: my initial assessment of HDMI in the current market is that it's used solely as a HD digital video signal carrier. For guys like me still deciding on an audio system, it also conveniently delivers audio to my display as well.