We do pretty high-volume newsletters and are using Silverpop now and Lyris/EmailLabs before that. Silverpop is OK (the frontend UI is kinda crappy, the API is functional but ugly, and the backend is PowerMTA which is pretty solid). They don't require double opt-in and give you control over the unsubscribe process if you want it. Once you start sending millions of messages a month, the pricing on MailChimp or Constant Contact (or SendGrid for that matter) doesn't really scale.
Silverpop was also recently hacked. I imagine email service providers make for easy targets. Email addresses are easy to sell, but aren't typically protected as well as obviously valuable information like credit card numbers.
I really like Mailchimp so I don't want the video to be taken the wrong way but a few of our challenges were with them.
Aweber (who we used first) on the other hand got hacked (twice!) and did not bother to mention that publicly. They did the first time, http://www.problogger.net/archives/2009/12/22/aweber-makes-a...
We've moved to Sendgrid.