i dont understand why people arent impressed with SB. It will be better than the historical c2duo, and then something else will come along and make SB obsolete. It is a natural evolution. Generally speaking, people desire newer processors eventually. I am not using my old PB G4 after all, though i loved the form factor.
Yes, if you buy a MBA now, it will become "yesterday's news".
One should buy the computer if you need it, and if you don't need it, then dont. If you have the option, then you have to weigh what you have now to what your needs are.
Certain apple computers stay fast for a while. A 2006 model mac pro can geekbench at over 5000... the top laptops just reached that level last year. The MBA geekbenches at 2000-3000. That mac pro can be bought for ~1100 on auction sites.
The good news is resale value.
I am a corporate user, so for me, the slow processor is not as important as portability for travel. I have a mac pro and other higher power computers for the crunching at work. It seems like a lot of mac users are students or younger people. For a corporate user like myself, it is about productivity. I am less productive if I cannot take the computer with me.
Sandy bridge is probably not important for me - where my needs are on the road, with MS office and internet capability. Powerpoint/keynote use. However, the second the MBA is too slow for my basic needs I will upgrade. Usually that is about 2 years. And the cost of the machine, whether it is 1000 or 3000 is not important for my corporation. I produce 100x-1000x that in payback for the company. So the upgrade process is easy.
When I look at ultraportables, I think of the business user. If I were a home user, the MBP 13" offers so much more for a few pounds. And if I were a home user/student/ entertainment user, I think the new MBP that will come out soon with SB are probably the best bang for the buck.