I think a sound general policy withy any computer purchase -- but especially with the purchase of a Mac -- is that the technology you buy won't be cutting edge for very long. Machines are currently on the edge of an update, but rarely on the edge of a reduced price. In other words, is waiting for an update worth six months of a "I'm on top of the computing world!" feeling?
It wouldn't hurt waiting until WWDC to buy, but I think you're safe buying now. There's certainly no major change coming to the Macbook line anytime soon.
Take this for what it's worth -- I bought my PB and two weeks later the Intel transition was announced!