Ah - very good question. Let's talk about how we host our data.
Servers
Our server farm is hosted through RackSpace and/or Voxel and all their data center, operating system and backup security policies can be found here:
http://www.rackspace.com/downloads/pdfs/RackspaceSecurityApproach.pdf
http://www.voxel.net/about/policies
Security and our Databases
MailChimp supports SSL encryption throughout the application.
From a recipient data specific stand point, all accounts receive a unique database once your list reaches what we consider a larger size and then we DO NOT co-mingle data at that point in one "master" database. Should an account owner ever reach out to our support team for assist, a support member can shadow an account. Otherwise employee access on our end is limited. Read our privacy policy for additional insight:
Outside of list data, credit card information is encrypted and obfuscated, but nothing else is for performance and capability reasons.