Buy one now.
I would suggest buy one now, for two reasons.
Reason 1: It is such a great machine right now. You will love it. I have the 2012 11", i7, 8GB, 128GB, and it is truly fantastic.
Reason 2: Macs hold good value when you are looking to resell it when the 2013 come out. This way you get the enjoyment now of a truly great machine and also only a smallish hit when buying the new one.
Just my two cents