JGC has built GTK 2.6.0 and friends (GLib, Pango), which now reside in Testing, waiting for people to break them.
These are fully compatible with apps compiled against GTK 2.4.x, so there should be no problems. If there are, you know where to go.
GTK 2.6.0 in Testing
Dec. 21, 2004 - Judd Vinet