GIMP 2.9 development release now has a PPA (the next “stable” release will be 2.10)
There won't be a GIMP 2.9 stable release - 2.9 is used for development and will result in the next stable GIMP version, 2.10
See this article:
http://www.linuxnewshere.com.au/index.php/gimp/gimp-2-9-available-in-ppa-for-ubuntu-13-10-linux-mint-gimp-2-10-road-map.html
The article seems to suggest it’s only available for Ubuntu 13.10 … but if you check the PPA it’s also available for 12.04, 13.04, and even 14.04
Install/upgrade instructions:-
sudo add-apt-repository ppa:otto-kesselgulasch/gimp-edge
sudo apt-get update
sudo apt-get install gimp
Revert the changes:-
sudo apt-get install ppa-purge
sudo ppa-purge ppa:otto-kesselgulasch/gimp-edge
According to the feature preview here:
http://www.gimpusers.com/tutorials/whats-new-in-gimp-2-10
it now supports 16/32 bit per channel … that should shut up the often used argument the Photoshop brigade have, right ? 
Changes include:-
1.) 16/32 bit per channel support
2.) (Almost) Lossless color space conversions
3.) New tool: Unified transformation
4.) Heavily improved scaling quality: No- and LoHalo-Samplers
5.) More on-canvas previews
6.) Multi-core CPU / GPU support (for filters)
7.) OpenCL-accelerated filters
8.) GEGL based filters and plugins
9.) ICC v4 color profile conversion support
10.) Color temperature correction
Be Aware, I’ve not tested this yet … but I’m gonna 
If nothing else, check out the feature preview link for a pretty amazing picture of a fly ;D
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