We sent the intrepid Ryan Salerno across 20 New York blocks armed with an iPhone 3GS and an iPhone 4. He ran speed tests along the way, ferreting out any oddities. Downloads? Exactly the same. Uploads? Several orders of magnitude faster.
Let's take a quick step back. In terms of downloads, the iPhone 3GS and iPhone 4 trade blows like boxers, each besting one another in speeds here and there - without any sure rhyme or reason. In other words, the results look typical. Depending on their exact proximity to various towers and buildings, one phone or the other could squeeze a bit more out of their radio transceiver.
However when you look at uploading, the differences are striking. In our tests, iPhone 4 averaged uploads that were 10.8X times faster than the 3G. Given the distance travelled, that's beyond the limitations of any single tower or connection, and a bit beyond any simple explanation we have as well (processor speed and RAM, for instance, might explain some speed boost but probably not this much).
UPDATE: Yes, duh, it's due to the HSUPA chipset, which was lacking in the iPhone 3GS.
So we tested our iPhones in other locations, Chicago and San Francisco, comparing 3G and 3GS models to iPhone 4. We found that, while the iPhone 4 wasn't always 10x faster, it was consistently at least 5x faster.
I guess it's safe to say, the iPhone 4 may not be any faster for downloads, but for uploading pictures and videos? We'll all see huge improvements. And yes, all thanks to HSUPA, what's clearly the least recognised, best new feature in the iPhone 4.