I bought an Asus Aspire 1. I put in 2GB of RAM($40), Installed a OCZ SSD ($160), and bought a 9 cell battery for it($55), and put Windows 7 Ultimate on it (TRIM support for SSD's and free with a BizSpark subscription).
I now have a machine a bit thicker than an iPad that runs a full version of Windows 7 Ultimate, has a 14 hour battery life, and an extra battery with 10 hours on it. It's "Instant On" because I never turn it off, it just hibernates, and the SSD I'm using is crazy fast and sips battery juice through a cocktail straw. All for a bit more than $500.
Oh, did I mention that I can replace the memory, battery and Hard Drive without voiding my warranty?
I understand the appeal of a MacBook Air. I do. But the appeal of beautiful design, a glowing logo rules telling me to keep my hands off my own hardware... It's just not work the extra $500 for me.