On par to be the most popular OS outside of Windows (give them time), Leopard has stunned the folks that follow these sorts of things - netting 2 million copies sold in record time. Think Vista pace.
One great comment from the above MarketWatch story, pointing to the unique benefit Apple enjoys is this "...in the Apple ecosystem, there are lots of evangelists that play a large role in proselytizing for Apple." That's the power of word of mouth.
My first clue Leopard was going to be big was when the Fedex guy showed up at 10am literally running to my door to deliver my pre-ordered copy of Leopard. I asked him how many he had in his route and he said 10 - nearly 1/4 his deliveries that day. He also told me that Apple gave Fedex a waiver that morning - the orders which normally require delivery by 10:30 could be delivered as late as 1pm. Still, Mr. Fedex was running.
BTW, here is a personal word of mouth example: I asked my Dad to buy and install Leopard so I could help control his computer remotely via their new sharing feature. But he just left me a voicemal that he had some install issues (on a new iMac) And I still can't get Back to My Mac to work, either.