So, while watching a live stream, I noticed this about the hummer:
There are two versions of the Hummer in GTAIV. The H1 series [the models in all previous GTA games], and the new H2 series. H2 hummers are available as both civilian rides, as well as the police versions seen in a released screenshot. Not sure about the H1; I saw it parked in a parking lot, but it appeared to be a plain red one. The H2 is called the "Patriot", but it remains a mystery what the H1 is named.