GNOME 3.0 will be released in September 2010, and in the meantime, they will release GNOME 2.30 in March 2010, continuing their long-standing tradition of six-months releases.
Note that this release date for 3.0 doesn’t mean that 2.30 will be less stable than usual. On the contrary, this will help them integrate the changes that are ready for 2.30, while leaving the parts that are still rough on the edges outside of GNOME, as used daily by the users, until after 2.30 is out. This will solidify both 2.30 and 3.0 releases.
The idea of doing GNOME 3.0 was first seriously discussed in 2008, before focus areas were defined in 2009, alongside a plan to reach 3.0. Those focus areas include revamping the user experience, streamlining the platform and improving the promotion of GNOME. Compared to GNOME 3.0, GNOME 2.30 will see the iterative improvements and bug fixes that people have now come to expect from our 2.x branch, in addition to some preliminary work needed for GNOME 3.0.