720p @60hz is defined by ATSC as 18Mbps. However, if you watch movies on cable/satellite, they are likely *not* digitally remastered at 720p (like Terminator 2 Extreme Edition), but instead, merely existing 480i content that has been upconverted. Producing content for 720p requires rescanning the original film frames at higher resolution, which most TV stations are not going to pay to have done for the Monday night movie of the week. That is, if say, NBC wants to show E.T. at 720p, they don't license the original content from Amblin, rescan at HD resolutions, reedit, and broadcast. More than likely, they take a video or DVD feed and upconvert.
Moreover, on satellite networks like DirectTV (which I have) the compression is pretty bad, partly I suspect because of upconverted content, and partly because of the codec parameters they are using. In fact, on a channel by channel basis it is different. For example, Star Trek Enterprise (SDTV) on UPN is *horribly artifacted*
The only channels that have real HD content are thoses specifically produced for HD, with HD cameras or mastering, like Discovery Channel HD. If you buy the T2 Extreme Edition (only 6Mbps bitrate), and play it through an HTPC on your HDTV, it looks far better than what you get on terrestial HDTV, even though the bitrate is lower.
The T2EE just looks amazing on my Samsung 5805w and PLV-70. I never thought MS's VC-9 codec could be so good.