802.11 is still rare in most environments. That chip would also cost Apple more money to put into the iPhone. At this point, it's not worth the cost to them and I agree. I would love there to be 802.11n in the iPhone since there's waaaaay too much interference where I live, but I just deal with it.
The only speed increase you could see is on your local LAN (anything going between your phone and desktop). It would not increase downloads from the net assuming you have DSL/Cable (that is the bottle neck, not your connection between AP and phone).
I imagine they did not want to install N because it is still in draft stages. It would be foolish to put a draft tech in something that is very hard/impossible to upgrade.
Also most of the N devices that are being pushed out run on the the same 2.4Ghz range...
I think adding N would have purely been for marketing, no performance increase.