Disagree...I have both (1.6\4gb MBA personal, i5\4gb X201 work) and it's not cut and dried:
- the X201 is the clear winner for CPU-intensive tasks - as noted above you definitely see this with Flash performance though I assume with other CPU-intensive tasks (encoding, compile, modeling) it would be even clearer.
- however the MBA GPU wins for anything graphically intensive (ie games) and of course the MBA SSD crushes the X201 HDD. I know that Nvidia Optimus and SSD are available for the X201 but adding those options close the cost gap and, with regard to Optimus, significantly impact battery life and heat (though Sandy Bridge will probably change that picture significantly).
- Battery life in my experience is roughly equivalent - there is no way you will see 10 hrs on an X201 even with the 9 cell battery (which adds weight and footprint).
- where the MBA runs away is ergonomics and usability - compared to the MBA the X201 is a heavy, clumsy brick. Unless you are a Trackpoint devotee (which I am not) there is absolutely no comparison between the Apple and Lenovo ergonomics - while the Thinkpad keyboard is great IMO the MBA's is better and the Thinkpad trackpad is useless compared to the Apple...I can't tell you how many times I have wanted to throw the X201 out the window because of a jumping pointer, shadow click or unsolicited window close or mail send...this just never happens on the MBA. However, the much more significant difference is form factor - until you use it there is no way to describe what a pleasure it is to carry and use the thin, light MBA compared to the thick, comparatively heavy X201.
It's interesting that the OP is considering running Windows; I've considered doing the same because I want to ditch the X201 and OSX support at my company is relatively limited...also, I'm not an OSX fanboy, to me Win7 x64 is at least an equivalent OS in many ways. What's holding me back is the relative crappiness of Boot Camp HID support and power management (though it would be nice to get TRIM support for the SSD). I haven't looked at Fusion or Parallels but I can't believe they would be viable alternatives for heavily CPU or GPU dependent apps.
Bottom line, if you have a dependency on CPU-intensive Windows apps go with the X201 (though even there I would wait for the Sandy Bridge announcements), any other scenario I would look very hard at the MBA.