As the development version of gimp is getting a significant amount of attention, I thought it would be worth giving a quick rundown of what's needed.
First a few caveats:
1) The 2.7.0 tarball on ftp.gimp.org is too old to include many of the interesting things like the single window mode, so don't bother with it.
2) This was done on a box running 2009-testing so it is possible that there may be (different) version issues, with 2009.1.
3) gimp 2.7 is in development, things may be broken, it's not a good idea to rely on it as your only graphics editor.
Step 1: Ensure that you have a development environment installed (and I may have missed a few things here, if you find such please add them as replies, but everything not explicitly listed as needing to be built is in either the main or the contrib repositories):
sudo pisi it gcc make autoconf automake gnome-build git libgnomeui libpthread-stubs
Step 2: Remove your current gimp installation (you can restore it later)
sudo pisi rm babl
Keep a note of all the dependencies that were also removed.
N.B. It may be possible to avoid this step but the configure scripts got confused about babl & gegl versions until I did it.
Step 3: Get the sources you need:
The current tarballs for babl & gegl on ftp.gimp.org work as of this posting, so download those & unpack them.
For the gimp itself you need the latest git version. To download this use:
git clone git://git.gnone.org/gimp
Step 4: Make sure pkg-config will be able to find the libraries etc:
export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:$PKG_CONFIG_PATH
Step 5: Build babl:
cd <where you unpacked babl>
./configure
make
sudo make install
Step 6: Build gegl (same formula in the gegl directory)
Step 7: Build gimp:
cd <where you downloaded the gimp git sources>
./autogen.sh # You may get a warning that libtool is possibly too new -- it's not.
make
sudo make install
The executable is /usr/local/bin/gimp-2.7
Step 8 (optional): Add gimp-data-extras.
git clone git://git.gnone.org/gimp-data-extras
cd gimp-data-extras
At least with the 2009-testing version you will need to edit autogen.sh as it doesn't know about automake 1.11, the quick & dirty way is just to replace "automake-1.10" with "automake-1.11" everywhere it occurs.
./autogen.sh
make
sudo make install
Step 9: Restore the distribution gimp (optional)
sudo pisi it gimp ...
Where the ellipsis represents anything else that was removed in step 2 and is not a dependency of gimp (e.g. extra plugins, translations etc.)
It is always possible that at some stage the git versions of babl & gegl may be needed.