According to Apple at least.
Apple had an event today to roll out their new MacBook Pro, their 1st sub-$1000 laptop, a new super-terrific Nvidia graphics engine and a bunch of other Apple-esque innovation around the edges.
According to Apple execs, as quoted by some after-hours Wall Street research, the Mac now represents 17.6% of unit sales in the US (wow if it's that high). If you look at “revenue share”, because Apple “focuses on fully featured systems”, they actually have 31.3% (double wow etc.). Further, Apple has 39% unit share for notebooks in education, surpassing Dell.
If those numbers are anywhere near correct that's a huge move from the low single digit market share Apple had for much of the last couple of decades.