IIRC, HDTV penetration was sitting at (roughly) 10% in 2005 when the XBox 360 launched and, (sometime in 2007) after most electronics stores stopped selling CRTVs, they projected HDTVs to hit 50% penetration in households in 2010. Since then the economic downturn has had an impact on sales of high-end electronics but it is uncertain how this has impacted projections of HD adoption. On top of this, historically a large portion of videogame consoles have been placed in a rec-room/den which tends to have a households second television which would be less likely to be a HDTV than their first console.
When you combine all these factors, it is unlikely that more than 1/3 to 1/2 of HD consoles this generation are hooked up to HDTVs; and it probably won't be a major selling feature to the majority of console owners until 2011 or later.