It appears from a DigitalPoint Forums thread that some Google AdSense publishers are allowed to place ads within email newsletters or other emails, with permission from Google.
Google explicitly does not allow ads in email.
From the Google AdSense Program Policies:
No Google ad or Google search box may be displayed on any domain parking websites, pop-ups, pop-unders, or in any email.
But it appears some publishers, with special permission from Google, are allowed to place AdSense ads in their emails.
Forum discussion at DigitalPoint Forums.