<p>Has anyone found any documentation or research about what data is transfered to Google Analytics when it's added to a site. The main thing I'm wondering about is post data, but the details of exactly what is sent would be useful.</p>

<p>I'm considering implementing it on a sites that have a lot of private data on them. I'm wondering what data Google will capture, if any. (The sites are login only.) I'm needing proof so I can provided to the users.</p>

The official information can be found
here
The visitor tracking information that you can get in the Google Analytics reports depends on Javascript code that you include in your website pages, referred to as the Google Analytics Tracking Code (GATC). Initial releases of the GATC used a Javascript file called urchin.js.
That script is then discussed in detail in
that blog
, and Google Analytics Help group can also provide
some details
.
a More detailed list of what that javascript collect is
listed here
.