The details are still emerging from this GNOME conference taking placed in Istanbul, Turkey. All the information that has reached the Internet so far are several GNOME developers briefly mentioning it on their blogs (aggregated through Planet GNOME). The only real information that has hit the blogs so far is that GNOME 2.30 = GNOME 3.0. This was mentioned on Vincent Untz's blog with a photograph. As of yet, no GNOME 3.0 information has appeared on any of the GNOME mailing lists.
Permitting that the GNOME development community will be continuing with their six-month release cycles and they plan on having GNOME 2.26 and GNOME 2.28 stable releases before GNOME 2.30/3.0, this would place their effective release date for GNOME 3.0 in early 2010. In fact, it would be right before the release of Ubuntu 10.4 LTS.
Meanwhile, the next GNOME release is GNOME 2.24 and its release is targeted for September. When we have any other information on GNOME 3.0 we will be sure to pass it along. Share your hopes and expectations for GNOME 3.0 in the Phoronix Forums.
UPDATE: With GNOME 3.0 will come GTK+ 3.0. Lucas Rocha just blogged about GTK+ 3.0 / GNOME 3.0 and that the transition process from GNOME 2 to GNOME 3 is expected to be smooth.