A little analysis that I did.
I bought my first Mac - a Macbook Air 11" in February. It was the time I got a new job and I wanted to give myself a little reward on the spot.
I'm kind of a hardware freak and I can't stand to have last years model, so when the Sandy Bridge models came out, I bought a 13" Ultimate in early August.
I kept the 11" for a while as I loved the small form factor and preferred if for some situations like going to the cafe, but after a while I realized it was really silly having two Airs so the 11" had to go.
I just sold it, took a big hit on the price, but I figured that it cost me $50/month in depreciation to own the 11".
With this in mind, I'm perfectly happy to spend $50 a month and always enjoy the latest technology. That's why I've decided to upgrade every year, simply because the resale on Macs are so good. I had no trouble selling mine on eBay in one week.
So, no matter what I think you could buy one now and reasonable expect to sell yours and upgrade later if you felt the need to.
The key is to keep your machine in perfect condition. People love the beauty of these machines and a scratch or dent can take hundreds off of the value. I'd also avoid Apple Care as you won't recover the value and you are covered the first year anyway.