The 66% would not hold if internet or corporate sales are included. Therefore your title exaggerates apples market share in computers over 1,000 by at least 2x, possibly more (I don't know the exact numbers.)
But, I suppose it is interesting that Apple stores are an effective vehicle for selling expensive computers. It seems kinda like a Mercedes dealership in that sense...make the process of buying the computer (or car) an experience rather than just handing over the credit card and walking out with a box filled with more crap to put in your house.
The high end share for Apple is growing rapidly, especially in the consumer market. Considering that the business market is arguably even more price sensitive (no citations, but I believe I've read this various places, citations to the contrary welcome), Apple is siphoning off the most profitable segment of customers, leaving HP, Dell, and the other PC players to fight over the segments with razor thin margins.
I'd rather be holding AAPL, which I am :). Just a few remaining shares, but I'm not going to swap them for Dell or HP anytime soon.
It also means that most people who are desirable customers (spend over 1,000) who buy for themselves and look at a product first pick apple. Thats huge. Which means apple had won the hearts and minds battle and just has to wait for these effects to trickle through the high end market.
The most common reason I hear for people not buying Macs is not having over $1000 to spend on a computer.