solipsism: That is a point many from other countries don’t readily see when they talk about their coverage, bandwidth and mobile plan costs. The US’ average population density is quite low, even if you remove the areas that aren’t inhabited or are agricultural, but leaving major road coverage. What country are you from?
I'm Swedish. Sweden is a country of 9 million with about 4-5 bigger carriers. They all use GSM so the iPhone is now available on pretty much all of them. The biggest carrier, Telia, had the exclusive for the 3G version. They support EDGE as well, a nice fallback in my opinion if 3G isn't available.
Other carriers offer great 3G, but lacks in GSM performance..
But since 3G hogs the battery like crazy I only enable it when surfing is on the agenda.
The carriers also have different settings for 3G and GSM on the iPhone itself. I can turn off my 3G but my friend with another carrier cannot, as his carrier prefers the subscribers to always use 3G since that's there main network. Ergo, I get a longer battery life. He gets a bit cheaper contract, but not always.
It's a coin toss.
I pay 100 US a month for unlimited voice and data. Well the data limit is 5GB, then they will limit your speed until the next month. I get maybe 3-4 days out of the battery (3GS). So very good coverage all around.