You make a good point, joechung, and I've been trying to find the related statistics. This is what I've been able to come up with so far:
Apple had 14% of the US retail PC market in Feb. 2008 -
http://www.appleinsider.com/articles/08/03/17/apple_snags_14...
This article claims the number was 8.7% in the 2nd quarter of 2009 -
http://www.appleinsider.com/articles/09/07/15/single_digit_g...
According to this article on theappleblog.com, Apple has 4% of the global PC market as of July 2009 -
http://theappleblog.com/2009/07/25/does-apple-have-a-91-perc...
If anyone has more, I'd be very curious to see them (especially the ratio of sub-$1000 retail PC sales to $1000+ retail PC sales).