I just want to know if there is a plan to replace Evolution with Thunderbird until 12.04
Thunderbird is now the default email client, starting with 11.10 Oneirc Ocelot.
Currently the etherpad from the session  lists these high-level requirements:
- styling improvements
- CD space
- e-d-s/calendar integration
- upgrade reliability (enigmail + binary extensions)
This decision should be made sometime around the Alpha period of 11.10.
Oneiric (11.10) has been released few days ago. Now it's a fact: Thunderbird is the default email client.
Take a look here. Oneiric (11.10) could be first step for Thunderbird to become the default client.