I should have done exactly this last year. Now I hesitate to buying a new Air as it might get an upgrade soonish...
There is still hope of the Nvidia 320m making its way into both the MBA and the 21.5" iMac. It is actually advantageous for Apple to use the Nvidia 320m in all of its MacFive* products right now. Since the 13" MBP, MB, and Mm all have it, the MBA and 21.5" iMac could be next to get the Nvidia 320m. I still say it's possible that the MBA will get the Nvidia 320m in the next month or so. I suppose that the 21.5" iMac should get the Nvidia 320m either way, so it's possible Apple will update some more Macs this Summer.
The iMac needs an update already. The Mac Pro is LOONNNNNGGGGGGG over due. And the MBA is seriously challenged with its 2 GB RAM and throttled CPU/GPU combo - not really having an update since October 2008. I suppose it's more likely that Apple will not update the MBA with the Nvidia 320m, but it will introduce its strategy for the MacFive with the MBA being the first to get the tech CPU/GPU that follows C2D/Nvidia GPU strategy. I would guess that October or January seem the most likely dates for that type of MBA update.
Intel is done making Core 2 Duo at the end of 2010. Although there's always the possibility that Apple stockpiles 15m C2D CPUs so it can keep selling its Nvidia GPU/chipset model in the MacFive for the next year or so... never know with Apple... it sure likes the Nvidia GPU/chipset model, and so do I!
Intel is really screwing customers and Nvidia/Apple by not honoring Nvidia's license to make GPU/Chipsets for use with Intel CPUs beyond C2D. It seems pretty obvious that Apple isn't likely to stick us with an Intel GMA as the sole graphics solution for any of the MacFive computers. Jobs did come out and give us an indication that Apple is disgusted with the GMA and they will do whatever it takes to ensure its MacFive users don't get stuck with only the GMA for graphics.
*MacFive refers to the 13" MBP, MB, Mm, MBA, & 21.5" iMac which all use the Nvidia GPU/Chipset model with C2D CPUs.