I just bought a 13" base air and I couldn't have been happier. Even if Apple does come out with a design change, my Air looses about 200-300 dollars in value. Seeing as how new airs will not be released until Q2 or ever Q3 2012, its a fair depreciation if I do want the latest and greatest. Since you are buying refurbished, you would lose even less. Don't think about it too much man, sure it's a big purchase, but this baby is gonna keep you happy and satisfied for a long time 😉
Oh and just my opinion, but there's no way a 15" air is going to be released. Why? Because the 13" MBP already faces a tough competition from the Air. And after the introduction of Ivy Bridge, the gap blurs even more. I think we will continue to have the 11" and 13" airs and 15" and 17" Pros. If you DO want a pro however (you haven't mentioned what you intend to do with your machine), I would suggest waiting for the new ones as there are wide spread rumors about a complete redesign of the pros.