I found a weird file added to the Google Docs web site, conversion.js

http://www.googleadservices.com/pagead/conversion.js

Digging through the code I found what looked like pixel tracking and links back to Google ads. Could this be the start of contextual advertising much like what we find in Gmail?

More details found here.
http://topdawgtech.blogspot.com/2006/12/where-to-place-ads-i-was-checking.html

conversion.js is used in a lot of Google services for stuff like downloads, sign-ups etc. Google also use Analytics a lot.

Conversion tracking is used for landing pages to monitor perfomance. Google advertises D&S, so maybe that's the explanation.

So no ads for Google Docs yet.

To give you an example, check:
http://picasa.google.com/