Apple doesn't have to use the Intel IGP even if it's there, but I assume it's still drawing power?
Obviously it'll draw some power, the entire chip can't be gated off since the memory controller is on the same die as the IGP. That said the IGP does downclock/downvolt automatically and if running on a discrete GPU you'll free up the majority of the IGP power draw.
However, due to the nature of turbo boost - which applies to both the CPU and GPU - that means the CPU will likely run on higher turbo-modes more often since it'll still be within the TDP allowance.
In short, running on a discrete GPU will net you back most of the power but not all.
I think anyone believing the Intel IGP solely would be sufficient in the MBA doesn't need to use an MBA as their primary computer OR has no care in the world about graphics performance. Going Intel IGP is going to be a 60% DROP IN PERFORMANCE over the 15 month old Nvidia 9400m.
Disregarding for a moment the fact that anyone using an ultralight as their primary computer have very insignificant computer needs to begin with, especially graphics-wise, you're post is too much FUD and too little reality.
Lets say it's 50% slower, numbers don't quite support that but it's close enough. 50% slower in what exactly? Synthetics? Sure. Reasonably modern, non-casual, games? Absolutely.
Do keep in mind we're talking about the difference between 7-10 and 10-15 FPS here though, it's not like using the 9400M would suddenly make Crysis playable on your MBA.
Indeed, if gaming is your primary concern why are you playing on a notebook to begin with? Let alone an ultralight notebook built by Apple, that's hardly a good mix for graphics performance.
For the vast majority of tasks one could want a MBA for you'd notice absolutely no difference. It's not like using an Intel IGP instead of the 9400M will suddenly make your window scrolling twice as slow or anything like that.
I have a hard time imagining Apple accepting Intel IGP in any Mac as a sole solution as the whole point of Snow Leopard is to take advantage of things like OpenCL and Grand Central.
Intel offers OpenGL support in their Windows drivers and are quite strongly pushing OpenCL as well. Both of those fact are quite irrelevant though as it's up to Apple to come up with the relevant MacOS drivers. It's by no means impossible, though I'm willing to believe that Apple would rather continue pushing severely outdated tech than write new drivers.
Intel is forcing computer manufacturers to use its IGP, but it's anti-competitive in nature. In the long run, this will not be a successful strategy. Until Intel loses in court, it will strong-arm pc manufacturers to buy the whole system. Nvidia will get another chance to build chipsets, and we are all losers by Nvidia not being able to produce for the Core series CPUs.
It's pretty much irrelevant whether or not Intel lose that particular case as far as the IGP is concerned. Even if nVidia could design chipsets for Arrandale what would that constitute, a southbridge?
By including an IGP, and a very competent IGP at that, on-package Intel has pretty much killed off any
reason to create a third party chipset. It's going to be cheaper, and more efficient, to just include a switchable discrete GPU to complement the Arrandale CPU/IGP and the Intel southbridge than creating a full chipset replacement that duplicate existing functionality.
The courts can rule in favor of nVidia as far as their chipset production is concerned but they can't dictate that Intel must not include an on-package IGP in their CPUs.
Indeed, CPU and GPU has been slated for reintegration for some time and you won't see any future Intel chips without it either. Sandy Bridge, the new Intel architechture aimed for late 2010/early 2011 further integrates the IGP by moving it on-die. This will reduce power draw further and help the CPU by reintegrating the memory controller back into the die as well.
Apple may well choose to stick to Penryn for the MBA and low-end MBP but rest assured it's due to appearances and laziness on their part more than any actual drop in performance. For the MBA and MBP 13 an Arrandale-class CPU would be a much more significant upgrade than moving from the 9400M to the Arrandale IGP would be a downgrade.
The vast majority of users simply wouldn't notice any difference.