OMG! Ubuntu! posted a news story that Ubuntu 11.10 will ship with Thunderbird as default mail app
Come October and the release of Ubuntu 11.10 Thunderbird will be installed by default. It will boast integration with various elements of the Ubuntu desktop such as Messaging Menu presence, Unity launcher badge and Application Menu support, all out of the box. Evolution fans don’t need to worry as their preferred application will remain easily installable through the Ubuntu Software Centre.