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.