Take a look here at the teardown for the late 2010 macbook air, the one with 320m graphics chip.
MacBook Air 13" Late 2010 Logic Board Replacement
In the last photos i can see 8 ELPIDA chips in a row... those are the ram chips... they are DDR3 1066mhz spec.
They are double data ram so they use both channels. So four of them are 2gb on one side are one channel and four of them are 2gb on the second channel. That means that for the 4G ram version for example they use 512mb per chip, for the 2GB version that would mean 256mb per chip. The problem it seems is that most memory sticks have 8 chips per stick with a total of 16 is you count in most laptops have two slots to put the memory in.
It would appear that Apple has chosen to use "single sided" sticks with just enough room to put 8 chips instead of 16.
For those willing to unsolder... 2GB version is most certainly upgradeable to 4GB if you unsolder all the chips and solder in double density ones.
NB. There is one more thing... there is another smaller chip called the SPD which stores information about the memory, speed, latency and so on. This might need to be changed too ... or overwritten with new data with something like Thaiphoon Burner. Or it might just work out of the box.
Now the problem with the 4GB version and upgrading to more is more stringent. First, you are limited my the number of chips so you can only increase density. In a normal situation, i know for a fact 320m will support two full 8GB of ram, and i've even seen 16GB ... i plan to get one pair to max out my mac mini 2010 (using the same 320m).
But with the Macbook Air essentially having space to use only half the amount of chips... just 8 instead of 16 you would need to get JUST ONE normal 8gb so-dimm memory stick (or buy a pair totaling 16gb and give one to a friend) and use it's chips... unsolder them and resolder on the mac. Make sure they are the same make and model Elpida, Hynix, Micron or Samsung of the original ones if possible.
This is risky, it might just not work, but you can probably resolder the old ones and be ok.
PS. Make sure you use an infrared soldering machine when doing it... you don't want to do more harm than good.