Apple probably could put "N" technology into the new iPhones but it's a case of whether it's worth it for them, since the N spec is a draft it could change, and they might want to wait until the iPhone necessitates the upgrade, since the "G" speed is probably enough for most of what average iPhone users do on their iPhones. All that being said though, with the focus of 3.0 seemingly being on video the upgrade to N speeds could be done at WWDC but obviously no one yet knows.
'N' is going to eat up way more power, as it requires an entire array of parallel antennae (the N protocol uses the multilink "MIMO" effect to achieve higher throughput) instead of just one.
But realistically, I think a 'G' pipe is more than wide enough for a mobile connection to those tubes of the internet. I personally would rather that Apple unlock the real CPU speed with OS 3.0 for older iPhones, giving us much faster page rendering.
802.11n eats up power like mad. Just because it's still in draft doesn't mean anything because, odds are, the spec is not going to change. There are too many draft N devices being sold.
FWIW, if you look at most web activities on the iPhone, it becomes pretty clear that data processing client-side is the issue, and not the network bandwidth. A desktop or notebook computer running OS X, Windows, Linux, etc, can generally render web pages on a mediocre network connection faster than an iPhone can on a good one, merely because it takes the phone so long to do the rendering.... So progress in things like multi-core phone processors and so on will probably make a bigger difference than 802.11n....
Besides that, outside one's own home, how available is 802.11n at this point? I didn't think hotspots support it very often. I haven't heard of many institutional deployments of n-class networks.