Officially Full HDTV is around 1080i, but can go up to 1125i as a mazimum.. Standard (SDTV) is on average 525 PAL/NTSC
As above a lot of the first 'HD-Ready' TV's are actually only capable of 720p (Progressive Scan) or even 480i (at Digital Quality).
The thing to watch out for is that with the upcoming HD-DVD and Blu Ray technologies, the companies have the ability to hardcode on to the disc a limit on HD output so that an older TV with HD capability would not be able to watch the movie in HD unless it could handle the full 1080i. This is being pushed as a method to prevent piracy more than a way of getting you all to buy a new TV, but the truth is yet to be seen.