I just "downgraded" from a newer 42" LCD 1080p to a new 50" PDP at 1080i. My screen sized increased, but my resolution actually went down. In my opinion, 1080p at anything below 50" is definitely not needed, unless you are a hardcore gamer (and even still, at below 40", it's totally useless).
I can notice the difference in the menus on my PS3 (small text) at 8 feet away or less, but most games actually look almost identical (and one looks BETTER) at 1080i. My Blu-ray movies look about the same -- very slight difference. I find the LCD did a better job at the ultra-fine details, but honestly, I think that's due to the sharpness of LCD, and not necessarily the fact that it's 1080p.
My LCD was so detailed, it was cool -- but sometimes unrealistic. People were so sharp, the image sometimes looked, "digitally painted." I switched because two LG LCD's I had were clouding in the blacks (drove me insane), but I loved it otherwise.
Of course, standard def movies look the same resolution-wise. My 50" PDP seems to do a better job with standard TV signals, as well (not so flat and pastel-looking like my LCD was).
I think that most people aren't going to care much about the difference. At 10 feet or more away, I can't see any difference in detail (other than slight LCD/PDP differences). If prices were almost identical, I'd go 1080p at 50" or above. Unfortunately, prices for 1080p sets larger than 42" are a lot higher, so it's still not worth the minute differences to justify going 1080p over 1080i.
And if you watch only movies, forget the 1080p unless you have an image larger than 47", and a big budget. Waste of money, IMHO.