Paul Thurrot made a few test a while back:
Firefox (any number of windows/tabs that derives from it) = 1 application.
Any explorer derived window (Internet Explorer included) don't count.
Many built-in apps don't count. (calculator, notepad...)
Daemons (background/taskbar apps) usually don't count.
It seems there is both a white list of "free apps" and a scheme to define if a new process is a part of an existing app.
Some manufacturer leaked that Windows XP cost around $15 per netbook. The point would be that "Windows 7, crippled edition" would be sold very cheap, so manufacturers are happy, and it's still windows, so consumers that want to run Office 2007 are happy.
Another thing to consider is that if you want to ship a netbook with Ubuntu (free) that plays videos/music, you'll (at least in europe) have to licence codecs. At the official store (http://shop.canonical.com/product_info.php?products_id=244) , A/V codec for Ubuntu cost $36, (only audio codec are sold for $26).
Windows XP includes those codecs (except mpeg2/h264)
Windows 7 include them all (including encoding)
So, from a manufacturer point of view, Linux may be as expensive as Windows, with the added fear that consumer will complain because they wan't install their favorite messenger.