I have 2nd Gen MBA (1.87, 128GB) and would jump on the new one for the 4GB of RAM if I could upgrade the RAM and processor on the 128GB model. I have over 50GB of free space as it is and have every program I could possibly need installed (including xcode which I installed and only used one time). I need a little extra power and RAM to run Parallels and would shell out the money for the new model, but don't want to pay $300 for an extra 128GB of storage I don't need. They need to add the processor upgrade option to the 128GB model.
If the last generation was any guide, the 2.13GHz model throttles down a bit because it runs hotter, so there isn't much difference between the 2.13GHz and the 1.86GHz most of the time. Last year, Macworld actually released specs showing that the 1.86GHz with SSD released in November 2008 actually slightly outperformed the 2.13GHz SSD released in June 2009.
That said, I did notice that the processor upgrades are available only on the "higher storage" models (128GB 11" and 256GB 13" models). However, you can purchase any of the new models to 4GB for $100 more. You need to order it with 4GB. You can't buy a 2GB model and upgrade it later.
Also, interestingly, the Boot Camp drivers that ship with the new MacBook Airs support only Windows 7 (in 32-bit and 64-bit mode). They don't support XP or Vista.