if you are running an old version of Thunderbird and want to try outall the new features 
that will be available in 3.0 - and feel that youcan't wait, well this week is the week to do
 it !You can play it safe and choose to update to 3.0b4 or even better to anightly which 
fixes a bunch of bugs. We'll provide some tips and support(1). 

In exchange we'd like to gather some feedback and find bugs due to upgrade.
https://wiki.mozilla.org/Thunderbird:QA_TestDay:2009-10-15 has thetechnical gory details :-)

(1)http://www.mibbit.com/chat/?server=irc.mozilla.org&channel=%23tb-qa