A regular old CRT(cathode ray tube, the classic TV) uses a display of 480i. 480p is enhanced definition, progressive scan on DVD players. Most HDTVs use 720p, though 1080i and 1080p are getting more common. When an HDTV takes in a signal, if it is not the native resolution, the TV has to convert it to the native res. How well the TV converts is where the lag can come into play.
You can reduce the chances of lag by having the imput from a XBox 360 or PS3 set to output at the native res of the TV. Both the 360 and the PS3 can output any resolution from 480i to 1080p, the media is what will determine what the max is. While this reduces the chances, some HDTVs(samsung DLP sets have a very bad reputation for this for instance) will lag regardless.