I suspect many people were very happy to see that the iPhone 4 was listed as having 802.11n. I know that I was. I have a dual-band network at home which separates 802.11n from 802.11b/g, and that's made a huge difference in transfer times. But there's also another benefit of 802.11n: it can use the 5GHz frequency band instead of the 2.4Ghz. Without going into too much detail, most (but certainly not all) of the benefit of 802.11n comes if you use 5Ghz, because it isn't cluttered with other devices like 2.4Ghz is.
So imagine my dismay when I went to the iPhone 4 technical specifications page and saw "802.11b/g/n Wi-Fi (802.11n 2.4GHz only)" listed there. I wanted to double check my suspicion that this was bad news for people who want to avoid the interference issues that come with 802.11b/g.
When you have a question about Wi-Fi, especially in the Mac world, the guy to ask is TidBITS & Seattle Times columnist Glenn Fleishman. If you've ever read a great, informed (and informative) article on Wi-Fi that even a mere mortal could understand, chances are pretty good that Glenn wrote it. He wrote the most thorough analysis of the Wi-Fi woes at the iPhone 4 demo that I have seen anywhere.
I asked Glenn about this on Twitter, and he confirmed my fears that 802.11n in 2.4Ghz will suffer from the same interference that b/g devices do. This makes "802.11n" partly just a marketing bullet point for iPhone 4 instead of a truly beneficial feature. If you're currently running a 5GHz-only N network, your iPhone won't take advantage.
For 2.4GHz N networks, you will probably get some speed boost over G (Glenn estimated perhaps 1.5x when using an 802.11n base station). When copying a 940MB file over my home Wi-Fi network between a Mac desktop and laptop, 802.11n took 2 minutes, 14 seconds compared to 15 minutes 45 seconds on 802.11g. Then again, most people won't be doing large wireless file copies to/from their iPhones until/unless wireless synchronization comes along.
So yes, Apple can boast that the iPhone 4 has 802.11n and they're telling the truth, but don't expect much practical difference day-to-day. Glenn suspects that given the newer chip and more efficient use of the radio spectrum, we might get better battery life on N than on G -- but we won't know for sure until the iPhones arrive.