Basically, it depends on what your needs are. If you actually need what the MBA has to offer, buy it now, and, as simsaladimbamba has already pointed out, you can always sell it if something much more desirable and powerful is offered next year.
However, if you simply want it, I'd suggest that you wait until the suggested updates have had an opportunity to have their early glitches ironed out, which could be a month or two after the updates are issued.
As the owner of a MBA, I will say that it is an excellent product, and I found myself using my MBP less and less once I got the MBA. In fact, it is now my sole computer.
Cheers and good luck with whatever you decide to do.