Ted has mentioned about new features in upcoming release of Flex 3 here.
I also found several references to AdvancedDataGrid and other cool features in FlexBuilder 3 here.
One feature I like the most is support to switch between different version of SDKs.
It is very helpful in development as well as production environment. I have been using FB from day number one in Macromedia when I started working on Flex.
Only complaint I have about it is that it is very unpredictable as to when it would take the longest time to build a simple project when you want it to take the shortest!