The MacBook Air uses an SSD, a solid state disk, available in 128 GB and 256 GB sizes. The current iMac has space for two drives, one of which can be an SSD, letting you put software you want to run fast, such as the OS, on the SSD, while using the physical hard drive for bulk storage. Other World Computing offers a range of replacement SSD's
for the Air which, at least according to them, are considerably faster than Apple's. For someone who has an iMac, would like to add an SSD to it, and is thinking of getting an Air, this raises an interesting possibility.
Get an air with a 128 GB SSD. Get the 240GB SSD from OWC, or an even bigger one if you are feeling extravagant. Replace the Air's SSD with the upgrade. Put the drive you took out of the Air into your iMac.
Cost (assuming you get the 240) is $479. That's $179 more than if you bought your Air from Apple with the larger hard drive—but you end up with not only a faster Air but a faster iMac. Buying your iMac with an SSD and a standard drive—admittedly, that would be a 256 GB rather than 128, but I'm not sure how important the difference is— costs $600.
I can only see one problem with this approach, assuming you are willing to pay the price to speed up both machines. Apple does not support third party installation of an SSD into an iMac. A little casual browsing suggests that it is possible but not easy, and I am not sure if at this point there are places that will do it for you, and if so what price.
But it is a tempting thought, for anyone who really likes fast machines.