Blender is a stable and production quality package yet it has been in heavy development ever since it became open source. Almost every version of Blender brought new features—some small and some very sophisticated indeed. The changes in Blender's development are neatly documented on the Blender site at: http://www.blender.org/development/release-logs/.
Blender's current, stable version is 2.49 and this version will probably stay around for sometime because Blender is no longer 'just' an open source package used by 3D enthusiasts but a viable production tool used in different parts of the production pipeline by professional studios.
Nevertheless, as of writing this book, the development of a new version of Blender is in full swing. The version number, 2.50, may lead you to believe that this is just a minor change but in fact, it is almost a complete rewrite. The most visible part is a completely different graphical user interface. This interface is almost completely written in Python, which opens up endless opportunities for writing sophisticated user interfaces to replace the limited possibilities in 2.49.
Not only the user interface is changed but also the internal structure is overhauled completely and although most of the functionality exposed in the Python API stays similar in nature, there are numerous changes in most of Blender's modules.
One major drawback is that the new version is developed alongside the Durian project (which will produce the open source movie "Sintel", see durian.blender.org), so the main development goals for version 2.50 are providing all of the functionality for this project. This does cover most issues, but some parts, notably Pynodes and screen handlers, will not be provided in the first production release.
On the positive side it will no longer be necessary to install a full Python distribution alongside Blender as the new version will be bundled with a full Python distribution.
The road map for Blender's 2.50 development may be found at http://www.blender.org/development/release-logs/blender-250/ but the timetable mentioned there is of course very preliminary. A full production version is expected somewhere late 2010 and will have a version number of 2.6.