The folks at ebizmarts have created a Magento plugin that synchronizes with your MailChimp list. It’s currently in beta, and can be found here on  MagentoConnect. Here’s what it does:

Once setup, the module will keep your Magento Newsletter subscribers automatically synchronized with MailChimp.

If you’re a Magento user, be sure to also check out our ecommerce360 plugin, which synchronizes with your shopping cart and shows your email marketing ROI.