PGR3 is on the cover of OXM this month
they were saying that they basically went to all the cities and took hundreds of hi-res photos (per city block) of everything to create there textures.
I have a feeling the games like Halo will have a hard time competing because the world is based in fantasy while PGR3 is based in 100% realism. In halo they'd hand make a lot of the textures while in PGR the can just take snapshots of the actual location.
Though I suppose Bungie could get creative and take pics of actual places that look similar to the effect they want, but then again it would make it harder to place in the fantasy world.