cyril7: For years now films have been Telecined (transfered to electronic format) as 1080p24 and then sub masters in 576i, 1080i50, 480i, 1080i60 etc are produced from that, it will be from one of these that a HDCAM copy in the US is sourced. Naturally there are "edited for TV versions" etc, again these will be derived from the 1080p24 master set aside as their own version.
Along the lines of what cyril was saying:
High-Def Digest HD ADVISOR: Regardless of whether a certain movie has been released on Blu-ray or not, the major studios have been mastering their movie catalogs in HD resolution for more than a decade now. In fact, most DVD editions are downconverted from HD masters. Sometimes, just one master will be struck for multiple uses in DVD, Blu-ray, or broadcast.
Many of the high-def movies in rotation on [USA] cable come from quite old and dated masters. In many cases, the studio may choose to withhold a certain movie from Blu-ray until they have a chance to remaster it in better quality. Unfortunately, seeing a movie in high-def on cable doesn't necessarily mean that a Blu-ray release is imminent.
Having seen broadcast HD rips of some movies I also have in my DVD collection and comparing them (eg. Star Trek II), it seems to me that the HD version is often the same as the source used to produce the DVD, like HD Advisor above suggests. In particular, Star Trek II was recently released on bluray with a brand new master that's received a totally different (and likely expensive) treatment to the old HD one, with a considerable improvement in image quality. I haven't seen that master used in broadcast yet though.