I agree, "substantially" was overstating it. However, trust me, population density is the lamest of lame excuses. Cell service is way, way better in rural Bulgaria then it is in New York City, where I live. Take an iPhone or other GSM phone, travel around Europe, and then spend a few days in NYC. It is all about the US being behind Europe in almost every category of infrastructure, be it cell service or high speed rail.
Coming from Australia, I've visited the US a few times over the last 10 years and I have to say it's improved substantially, though coming from a remarkably poor baseline.
2000: My 2G GSM phone doesn't work at all here, although I'm happily roaming worldwide in Europe, Bangkok, Singapore, etc. Instead, there's something I've never heard of called TDMA here. Meanwhile, my US Mac friends all have pagers, and think a mobile is a bit fancy ... WTF??
2002: My new tri-band 2G GSM phone now has a new mode that I can manually select to make it work in the USA. Huzzah! But nobody over here has ever heard of SMS yet. FFS??
2004: My quad-band 2.5G phone now automatically roams in the USA. W00t! In the US they still found this exciting, because I saw all the ads listing some with a feature, "worldphone"
2006: Australia has had a 3G network for three years but coverage was crap until now. I buy my first 3G phone, and lo-and-behold it actually comes with 3.5G and the networks already gave me HSDPA for free. As a result, I got addicted to Opera Mini browsing and mobile TV. Which is why I was a bit "meh" when iPhone first came out. They still don't have RTSP on iPhone 3G, so no mobile TV.
2008: I get another 3.5G phone, and I'm impressed to see rollouts of HSDPA and LTE actually coming to the USA. Australia still has huge swathes of towns and areas with no 3G (though this represents only 10% in population terms), except for the ex-monopoly carrier who has a clearly superior but costly 700MHz network that gives 98% population and excellent in-building coverage but requires special phones as a result.
Ironically, all my phones in this time were from Motorola. So it's not like the USA doesn't know what it's doing wrong sometimes ;-)
CK.