I've spent a long time trying to install GIMP 2.8 on my Linux Mint 12, now that I finally was able to do it I'm sharing with you all how to do so.
First open the terminal and type this command, since I'm on KDE I'm using Konsole
sudo kate /etc/apt/preferences
Then insert the following lines to the file, save and close
Pin: release a=oneiric*
Then on the terminal again run
sudo add-apt-repository ppa:otto-kesselgulasch/gimp && sudo apt-get update && sudo apt-get remove gimp && sudo apt-get install gimp
Not that hard, right? Let me know if you have any problems.