While it's likely that distros will want to drop support for GTK 2 as soon as they can, there seems to be some issue on the part of GNOME independent GTK apps regarding GTK3:
http://igurublog.wordpress.com/2012/11/05/gnome-et-al-rotting-in-threes/
and:
http://www.linuxuser.co.uk/opinion/a-linux-conspiracy-theory
Perhaps the most telling statement is:
Beyond just GNOME apps and tools being stripped of options, Red Hat employee and lead GNOME developer William Jon McCann was caught opening a bug report on the independent Transmission BitTorrent client telling the developers that its panel notification feature should be removed. Why? Merely because GNOME 3 no longer supports a panel: “Transmission has an option in the Desktop tab of the preferences to ‘Show Transmission icon in the notification area’. This should probably be removed.” Transmission developer Charles replied, “So now we can have three builds of Transmission that decide at compile time whether to use AppIndicator, GtkStatusIcon or nothing at all… Removing it altogether, as you suggest, will hurt Xfce users.” McCann replied, “I guess you have to decide if you are a GNOME app, an Ubuntu app, or an Xfce app unfortunately… And I have no idea what Xfce is or does, sorry. It is my hope that you are a GNOME app.” Charles’s reply to this: “*speechless*”.
I like Geany since it is light and works seamlessly with XFCE. Will the Geany be forced to choose between GNOME and everyone else by moving to GTK3? Please investigate this carefully.
- Nate