It's rendering at 1080p @ 60 Hz on my display, my display confirms it.
I don't think you've understood what people are saying. Just because it's in that graphics mode doesn't mean it's rendering at that resolution. Even in Blitz3D, I could run at 1600x1200, render to a 256x256 quad and scale that to the screen. Your "display would confirm" that it was running at 1600x1200 but it wouldn't be rendering at that resolution. And GTA IV clearly is not.
In general, I've noticed a distinctly muted response. Is everyone spending every spare minute playing it or are people a little underwhelmed after all the hype? Personally I think it's just like every other GTA game but a bit more depth, a bit better graphics, etc. So far, I haven't noticed any way to buy properties either, which would actually be a step down from San Andreas. There's nothing wrong with it, it's a decent game, but not the perfect 10 all the reviewers were claiming for me.