Plans to release GTK sometime next year are complete fantasies. It's also going to take several years (remember GTK1 -> GTK2 migration). In this aspect it won't be much different from QT3 -> QT4 migration.
I'll even make a prediction: by the time GNOME3 is released, it will be in the process of being abandoned in favor of KDE.
KDE4 was not just a significant ABI/API change, it was a major rewrite of most of parts of software. KDE 4.2 is fairly feature-complete now, with lots of new features compared to KDE 3.5.10.
On the other hand, you can count on GNOME to remove few features just to be "less confusing" to user.