In case you are wondering, the title was not a mistake. The GTK developers are actually planning to break the GTK 3.0 API intentionally. If you do not know, GTK is a toolkit for creating Graphical User Interface. GTK is one of the most popular toolkit for for the X Window System, along with Qt. While Qt is mainly used with KDE, GTK is used in GNOME.
Here is an excerpt from GNOME developer Jason D. Clinton's Live Journal entry:
Mattias Clasen talked about the plans for intentionally breaking the GTK API for 3.0 in order to make it possible to achieve new features: XInput2, the drawing API, and changing theme engines without breaking applications. Emmanuaele Bassi jumped in to help Mattias Clasen with explanations. A goal is to make it harder for people to do deep hacks like the current string of abusive theme engines that will break from release to release and provide a feature-rich foundation for doing work correctly upstream.
That is all way over the head for normal users. So, to the normal user, what it meant is basically two things. First, they are ready to break GTK 3.0 API in order to achieve new features and make changing theme engines easier. And secondly, they want to provide a foundation on which the theme engines can built on, so that they do not break every othere release.
In a way this approach seems like what the KDE developers did with KDE 4.0 and the Amarok developers with Amarok 2.0. Both of them were criticized for this approach initially but both has now developed solid product over it. The same is likely to happen with GTK 3.0 is they go ahead with this plan. However, in the long run they can really end up developing a great application, even though it may cause some inconvinience at initially.