Just published to WordPress.org: http://wordpress.org/extend/plugins/chimpblast/
ChimpBlast lets you compose broadcast email messages for use with the MailChimp service from within WordPress. You can compose messages in the WordPress rich text editor, import the content of posts, preview the results on your website, and then submit the resulting message to MailChimp using the API.
ChimpBlast was designed for use in combination with RSVPMaker. You can import your RSVPMaker events into ChimpBlast, add introductory text, and send out your event invitations. The RSVP Now link in the invitation will automatically be coded to include a reference to the recipient’s email address, allowing RSVPMaker to retrieve that person’s profile details so they don’t have to be reentered manually.
You can import a your MailChimp template and add CSS to help your WordPress content display better, such as the markup for WordPress photos and captions, or your own stylings. The MailChimp service automatically inlines the CSS code to work better with email clients, which don’t always respect CSS in the header.
This is what I use for most of my email marketing work. I’m a big fan of the flexibility of the MailChimp service, as well as the high standards it enforces for list quality (even though they sometimes make me jump through hoops to get a list cleaned up). I actually don’t like the visual editor they provide with the service that much and often would rather use the one within WordPress – particularly since the content I send out is often derived from a blog post or RSVPMaker event.
If you downloaded the preliminary version I published on RSVPMaker.com, please download the official release. Note that you may have to redo the email template. This version allows you to have several alternate templates available for use.