If you buy now, there might be an update in July, if you buy in August, there might be an update in September.
I would either:
A. Buy now
B. Buy after Macworld (January)
You could also buy a cheap notebook now, and then wait for the inevitable update without losing too much money and getting time to save some more. (And, you always have to remember that there is a chance, though so slim, that you won't like your Mac OS X experience, you don't want to be out too much money)