Mozilla has pushed out the second and final beta for Thunderbird 5 on to the Beta channel. Those already have Thunderbird 5 Beta 1 install can get the update by going to Help >> About Mozilla Thunderbird. The update should download and once completed click the Apply Update button. Upon restart go back to Help >> About Mozilla Thunderbird and you should see the about screen as shown above.
The final release of Thunderbird 5 is tentatively scheduled for June 28th. Thunderbird 6 will be released on August 16h.