You should also know that the new dual core P-M's will be much more expensive than single core ones when they first come out. It will take awhile for them to drop in price after they are first introduced. If you're looking for discounts off the current generation of CPU's, you probably wouldn't be interested in paying a premium for the newest technology when it first comes out.
Also, if you want an Asus laptop, they seem to wait awhile before releasing a model based on a new processor or platform. They won't be the first ones out of the gate with a new model based on a new platform, like Dell or Acer. I think it was 3 months after the Sonoma platform was launched that the Z71V came out. The Acer 8100 had been out since January; the Z71V launched in April. If that trend repeats itself, it certainly won't be this January when we see an Asus laptop based on Yonah or Merom. And that's not even mentioning the not so uncommon delays and/or shortages we've seen from Intel when bringing new processors to market.
So as always, the best advice is to buy a laptop when you need it. If you think you can wait several months and that the performance increase from Yonah or Merom will be worth the wait, then by all means, wait until the new stuff comes out. I can't imagine there being much of any discount for Christmas from anyone, maybe $50, if that. So if you need a laptop now, it's as good a time as any to buy one. But if you can wait, there's always better and faster technology around the corner.
And as far as where to buy from, ISTNC, Geared2Play, and C9Tech are all great places to buy from. You shouldn't have a problem with any of them.
Hope that helps