Well, looks like the "Digital revolution" for television has been delayed, which may delay the "HD revolution" as well. If its not mandatory, well, that's going to slow people adopting it, after all. "I can wait until later" being a fairly reasonable explaination for putting off getting a new HD television. So, its entirely possible that the many 360 and PS3 owners this generation might not see the best their systems have to offer. Similarly, this might affect the adoption of Blu-Ray and HD-DVD, since their main selling point depends on HDTV.
Provided HD costs continue to go down, by 2012 (7 years after the 360 debuted) hopefully it'll be a safe assumption that all the early console buyers will have HD, and by the time the new set of consoles really start taking it off that HDTV will be the defacto standard in homes.