There has been forum discussion on how AdSense is not allowed in emails at WebmasterWorld. In fact, here is the first known sighting of adsense in an email reported over at WebmasterWorld. In that second thread, they describe how one would add AdSense into an email. Back on March 25th, I wrote an entry on this named Adding AdSense to Emails, which points to a DigitalPoint thread on the topic.
But now, Jenstar at her blog JenSense writes an entry named AdSense in email newsletters for regular publishers. In that entry, she says that "Google AdSense has allowed a select number of publishers to run AdSense ads in their newsletters." She quotes an article at eWeek that confirms this:
Google plans to eventually expand the newsletter option beyond its direct AdSense partners to include the smaller publishers who use AdSense's online self-service program.