Hi Daniel,
Thanks for getting in touch! Gmail's tabbed inbox has been the topic of much discussion recently as ESPs and other senders attempt to learn how Gmail is filtering emails and determining which tab to route messages to. For more information, I highly recommend reading over the following blog post on from one of MailChimp's delivery engineers: http://blog.mailchimp.com/how-gmail-tabs-affect-your-email-marketing-an-update/ As covered in that post, it does appear that Gmail's tabbed interface has increased some measures of engagement with commercial emails. If users are seeing those messages go to the promotions tab and don't want them there, the best bet is going to be to encourage them to move those messages to the inbox manually. Additionally, adding the "from" address to the recipient's contact list can help encourage those emails to be passed to the inbox.
Not all emails sent through Mandrill get marked as promotions, but Gmail is the one that decides whether mail goes to a specific tab, and they use a variety of criteria to determine that. That criteria is not available to senders, and Gmail is subject to change their criteria at any time. As a result, while they can test variations of their emails to see whether certain content is causing it to be sent to the promotions folder, it's generally not possible to completely prevent that from happening. Tabs are a still a relatively new feature in Gmail, so we expect that the criteria can and will change as Gmail and Gmail users get more accustomed to the tabs.
Let us know if you have additional questions.