These sites were helpful to me to understand exactly what DNT is doing and what affect it will have:

http://ie.microsoft.com/testdrive/browser/donottrack/default.html
http://donottrack.us/

As a previous reply mentions, it's more of a preference flag to tell sites how to behave (which they can respect or not), rather than anything different in one's browser. It doesn't turn off cookies or javascript, for example, which would have a real affect on Google Analytics.