if you want to keep it for 4-5 years+ you should definitely wait, there's no place for a C2D in 2014.
Why? If your needs don't change, it really doesn't matter. Software that runs fine on a C2D now doesn't suddenly need a lot more CPU power in a couple of years. Current versions of software from 4 years ago still run perfectly fine on 4 year old computers.
The exceptions are pro-tools, software that really needs as much CPU power and memory it can get, and more heavy photo/video editing, compiling, etc. But if you require something like that, you will a) not be looking at a MacBook Air, b) know perfectly well you will need to update a lot sooner than in 4-5 years. So if you already know that the C2D now is not fast enough for your needs, buy a MacBook Pro and be prepared to upgrade in a year or two.
But with normal use, most people hardly ever stress their C2D CPUs and only use a fraction of its capabilities. That there's a new CPU model doesn't suddenly mean it old one becomes any slower. Turn on the Activity Monitor, do your things, and check the CPU usage. Sure, it might spike every once in a while, but overall it's sleeping most of the time. And this is true for most people. Even though everyone likes to consider themselves a pro-user, this doesn't mean they actually
need the best that might be soon available.
Most of the speed you need comes from the RAM (2GB is cutting it close when you multi-task a lot; 4GB is enough for normal use also with newer software the upcoming years) and the SSD. Why do you think that most of the people who use the Air don't notice that this supposedly outdated C2D is too slow? Because they only use a fraction of it. They just notice how lightning fast the drive is. And even if their RAM runs out, the SSD is fast enough so that swapping to it doesn't really seem all that different from simply having more RAM.
Bottom line: things change. The time that you needed to upgrade every year to run the latest software is long behind us. People tend to look at individual specifications, which is understandable, but it's shortsighted to not look at the whole picture. Apple knows that many people don't need an i7 CPU. But they
will notice a fast drive and proper GPU for video. And that's exactly what the Air packs. So, don't worry too much about having the latest CPU model. Instead, look at what you'll be using it for, and if you buy it just to have certain specs, or to actually get your work done.