Personally, even at 50 inches or more...the human eye really cannot differentiate the difference between 720p and 1080p...provided the display is accurate calibrated and is presenting source material at 1080p. The only time you really see a difference is when you are far too close to the display and pick out the pixel detail.
Another thing to consider is that if you watch a lot of SDTV...it's actually advantageous to have fewer pixels than more. Nothing makes a crappy signal worse then having more and more of it.
The only reason I would consider paying more for 1080p is if I planned to have the computer connected to the display on a regular basis.
Here is some corroborating information to back up my claims that you won't really see a difference
1080p-720p shoot-out in Athens! - AVS Forum
1080p and the Acuity of Human Vision — Reviews and News from Audioholics
Also, a TV with a better video processing unit running at 720p can often perform better than a 1080p set with less sophisticated circuitry.