As of this afternoon, the Mozilla team has made Thunderbird 2.0 available for download, this release coming before the end of the month as they wanted. You can download 2.0 for the various supported platforms now straight from the Mozilla site. The featureset hasn't been expanded over the RC releases, so you can expect the same if you've been using those up until now. Many new features have been added over the 1.5 series, such as message tagging, an updated UI, vastly improved searching tools, improved extension support and more. The Windows installer has been revamped and better support for Windows Vista has been included on those ports.
I've been impressed with the 2.0 RC series so far, and if you are a fan of Thunderbird it's definitely worth a look.