I ordered my iMac before the rumor of the early Intel iMacs came out, and honestly I would probably have waited if I'd had the information at the time - but I'm very glad that I went ahead with my order after I heard the news. No doubt that the iMac will get a significant performance boost with a dual core Yonah inside, but my previous systems were a) Windows and b) terrible, and for me the extra few months of use are going to be more than worth the tradeoff in performance.
If I was getting a laptop, however, I'd seriously consider waiting. In my (non-expert, uninformed) opinion, the Apple portable line is going to be absolutely transformed by the increased processing power and good energy conservation features available in the Intel mobile chips, and I'm almost 100% sure that my wife will end up with an Intel based powerbook under the Christmas tree next year.
I hate to suggest that someone else wait for a new Mac, when I just got my new one today (and am loving it) - but the iMac is likely going to just get a new processor, and a little faster - while the Powerbooks and iBooks are going to get a LOT faster, gain extended battery life, and enjoy the flexability of a lighter, thinner design. I am really interested to see what the engineers at Apple have come up with.