According to the blog of one G employee, it seems they will soon be making an attempt to share more information about how Gmail works and clear up some of the concerns that have been expressed re: privacy.
This employee states on his blog, for example:
It's really important to note that ads are served on a per-email basis, and that the ad targeting information exists only as long as it takes to actually serve the ad. As soon as the ad is served: Poof! Gone. It's not recorded into any sort of uber-profile, and it's certainly not passed on to any third party.
This is the kind of stuff they need to be explaining much more clearly than they have been so far.