Well it looks like you can get your security update for Thunderbird now to v1.5.0.10. Highly recommended to update to it.
As far as Thunderbird 2.0 goes, my research shows that March 6th is the final code freeze which means they’ll be getting into RC builds soon and then we’ll have a sparkling new Thunderbird to play with in March or April. 
Thunderbird 3.0 is due sometime in 2008.
