Maybe 720p is way to go, as I can get Samsung PN51D450 for $599 (non 3d, 720p) or PN51D490 for $699 (720p 3d).
We have 3 (Panasonic) Plasmas in the house: 42", 50", and 60."
The 60" is a new ST30 (1080p), the 50" is a 1366x768 panel, and the 42" (the oldest) is 1024x768. The 42" shows "obvious" SDE at any viewing distance closer than about 8 feet, but still looks pretty good from 9 ft & beyond.
With Blu-ray disks (or HD-DVDs...) the 60" Looks Great (the odd Green & Pink tinting notwithstanding). But at a 9ft viewing distance, ANY OTHER Content, including "HD TV" programming tends to show off the flaws in the source material. (The best HD TV material, via Uverse, does look pretty good on the 60 - but much of the "HD" programming really deserves to be called "pseudo-HD"!)
At 9ft or beyond, the 50" set - even though "only" 1366x768 - easily provides the best PQ for Upconverted SD DVDs, as well as any other SD material that we happen to watch, and still manages to look Pretty Darn Good with HD sources.
Would avoid the new 50" Panasonic "720p" sets, but we know several people with 50" 720p Samsungs, and they have been happy with them - although the screens tend to be rather reflective!
Still, if the price difference were within $150 or so, would be hard pressed NOT to select a 1080p set: but then its NOT MY MONEY!