V2.0 of the API was a weird hybrid of RESTful and RPC practices, which required everyone to learn our own special style of API design. With a few minor exceptions, API v3.0 is fully RESTful. This allows us to express in a couple dozen resources what it took the last version 120 distinct endpoints to accomplish.

http://devs.mailchimp.com/blog/api-v3-0-officially-launched/

Consuming this API doesn't seem to be that hard but I hope they'll release a PHP wrapper soon.