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.

Comments:
GuyFromChicago
09/18/2006 02:35 pm
It's doable if you have been approved. The biggest issue right now is that adsense won't display in a somewhat common e-mail app called outlook...
BizDevInSunnyvale
09/20/2006 04:40 pm
We have had AdSense ads in our newsletters for over a year now. These ads come in various formats and are essentially served by an image server. Publishers have to insert Google-supplied javascript snippet for that ad type.
cool
09/21/2006 04:15 am
how to get permission to display ads in newsletters?
blue cloud
09/21/2006 04:19 am
<a href="www.readerstuff.com"> readerstuff.com</a> what will google do if any publisher publish adds with permission.does google cancle its registration?
Science Guy
07/07/2009 11:44 pm
Anyone know any other programs to put ads in email newsletters, besides adsense? I've heard of Pheedo, but I don't think they are free. Also, I successfully put ads in my newsletters through menus I found i feedburner!! If google doesn't want that, I don't know how they left a way for me to do it...
Namita Desh2011
09/01/2011 09:21 am
can through feed burner will add ads in email i have this blog http://www.latestgovernmentjobs.in and i want to that blog rss feed content ads