The reason is rather clear: the GNOME team wants to ensure the quality of the release is up to snuff. To make sure that it is, the release date for GNOME 3.0 has been pushed back until September 2010, while the March 2010 release will be GNOME 2.30 (in other words, 2.32 will be 3.0). The March 2010 date was clearly preliminary, so a delay is nothing surprising.
"The release date for 3.0 was not set in stone: while we're using a strict schedule that allows us to release GNOME every six months, GNOME is above all using quality-based release engineering," Vincent Untz writes, "That's why our community wants GNOME 3.0 to be fully working for users and why we believe September is more appropriate."
GNOME 3.0 will be used as an "excuse" to cut out a lot of the cruft that has accumulated within the platform over the years, but at the same time, the interface is being redone from the ground-up. The interface is built around the new Clutter-based GNOME Shell and Zeitgeist.
GNOME 2.30 will not be a lost upgrade, though. "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," Utz writes.