One of the biggest disappointments of Mozilla e-mail client Thunderbird 3 was that plans to bake in the calendaring extension Lightning were abandoned. When Thunderbird 3 debuted, users got a bit of a shock: Lightning wasn't compatible with the upgrade. Although there was a work-around through the Lightning nightly updates, the official build wasn't going to graduate.
Lightning 1.0 beta 1 fixes that, and also introduces to stable build users features that have been available in the nightly for some time. Lightning now displays the calendar and accompanying task manager in tabs, offers support for SeaMonkey 2, allows you to set multiple alarms for one event, and improves CalDAV interoperability. The full change log can be read here.
The Provider for Google Calendar also hasn't been compatible with Thunderbird 3 until now. Although there are several add-ons that provide bi-directional Google Calendar access, the Provider is preferred for Lightning users because it offers seamless integration between the two calendars. Basically, the only reason not to upgrade Lightning is if you don't plan on upgrading from Thunderbird 2.