For those Thunderbird users who have not picked it up via the automatic-update system, Thunderbird 1.5.0.10 has been released. It is a security/stability update.

Thunderbird 1.5.0.10 release notes

Thunderbird 2 is not too far off, the current plan is a code freeze set for 6th March which will lead to a release candidate being made. The release candidate will then be subjected to quality assurance testing and assuming there are no problems the final release about 2-4 weeks later. These dates are subject to change and there may well be several release candidates if issues are found.