Apple has 91% market share in the US for computers costing over $1,000, but only 8% share overall

"According to NPD, in June, nine out of 10 dollars spent on computers costing $1,000 or more went to Apple. Mac revenue market share in the "premium" price segment was 91 percent, up from 88 percent in May."
Source: NPD, reported by betanews, 22nd July 2009
Overall market share:
"Apple's share of the US computer market fell to 8 percent during the fourth calendar quarter of 2008 from 9.5 percent in the third as the Mac maker surrendered its third place ranking to surging netbook maker Acer, according to market research firm Gartner."
Source: Gartner, reported by AppleInsider, 14th January 2009
(If you have any more up to date figures please let me know!)