Let me start by saying – it’s worth getting, and that I don’t think most people will experience my issues; be aware that I am talking about the game in that context.
I think what I’m seeing is probably part settings – I went from a dark, crappy, low-def 27in TV I was sitting 8-9ft away from, to an amazing Samsung 24in LCD monitor from which I am less than 3ft away with my 360 running in 1920×1080.
The other major factor, I believe, is some kind of cautious texture loading. If I could get a screenshot of my 360 (and, hey, Microsoft, how about you add Gamer Blogs which specifically provide a screenshot-to-live service that games can take advantage of?) I could show you exactly what I mean in seconds. Easiest explained in what I just saw: a car drives infront of me, side on, with a super-charger protruding thru the hood. The charger, hood and wheels are in exquisite detail. The whole side of the car is in ultra-low res, pixelated and nasty, yet overlaid with high detail lighting and whatever else that makes it quite jarring.
After the car drives past, I turn and run up to where it has stopped at a light, and now the whole car is in beautiful detail as I approach.
Like that first few seconds after a loading screen in Mass Effect, where the textures are all in their first LOD, and it takes 1-5 seconds to pop thru a couple of LODs into glorious spledidness.
Obviously GTA has a helluva lot more models and textures to deal with, so it probably spreads it out over a significantly longer time…
So, I fire it up, sit back a ways, and try to relax my focus for a while, and quit worrying about the artefacts and just enjoy the game – which is easy enough :)