I definitely notice the difference between 720p and 1080p bluray rips. Especially on good quality rips. Things like background scenery and broad, distant images are much clearer in 1080p guise. I've tried this 'test' with a 720p and 1080p rip of the same movie. Definitely noticeable imo. And this is from a comfortable viewing distance and not staring at the screen from like a metre away. But obviously the further away you are from the screen, the less noticeable the difference between the two will be.
I'm not sure about XBox but quite a few PS3 games I've come across are 1080p. I've been playing GT5 (which is 1080p) quite a bit recently and the clarity is unbelievable. Depending on what you're viewing, I think the HD vs FHD concept is rather important.
OK, I'm not a big gamer, so really can't comment on the gaming part. I did get the PS3 + GT5 combo now (as a 3D source for my new LCD), so will do some tests soon ...
Instead of comparing 2 different rips (one at 720p and one at 1080p) ... they may have completely different compression settings etc. I have previously done the following little test (like Postman said)... My AVR has two HDMI outs, so I was able to run the same 1080p feed from an Oppo BDP83 (so BluRays, not rips) to two panels, one 50" FullHD (1920x1080), the other 50" HD Ready (1366x768), but the last one was at least also capable of processing a 1080p signal. Viewing distance 2.9m.
On feature films I could not detect any difference in detail, honestly nothing. If you paused the pic and closely examined certain parts, sure, you could see differences, but could still not really tell which one was sharper. Feature films, together with their 24Hz frame rate tend to use bigger appertures on the cameras to have a bit of a softer picture and honestly you do not need more than 720p.
On some other material like nature documentaries (ie. Planet Earth) and some Classical music concerts, the cameras tend to give a sharper picture, and OK, on these having the sets side by side, you could detect very slight differences, but actually so small that if you had to do the test with the panels in different rooms, so you could only view one at a time, I think it would be very hard to tell them apart. Same goes for Pixar type movies.
So, maybe to say there is NO difference would be wrong, but I would say every other aspect of the panel is probably more important. ... If you're gonna get very close to a 50" screen, the dotpitch (correct term??) on an HD Ready might become an issue though, but really not the amount of detail.
That said, 720p sets have one huge problem (and I'm not joking on this one):
When someone sees your new TV it will always go like this:
Visitor: Hey, nice new TV. Is it FullHD?
You: Em, no, ... bla bla bla ... explain explain explain
Visitor: I see.
With a FullHD set you get:
Visitor: Hey, nice new TV. Is it FullHD?
You: YES
Visitor: Nice.
From viewing distance, it's generally accepted that you can't tell the difference between HD and FHD. Unless you're Superman... 
Even a real guru like TYR admits this. IIRC it was in fact him who did a comparison, and he could only notice the difference between HD and FHD while viewing still pictures, or something like that.
