GIMP 2.7.1 added Single Window Mode and GIMP 2.7.x can now be installed via PPA. It is available here for Jaunty, Karmic and Lucid. I have Ubuntu 10.04 Lucid Lynx installed and I installed GIMP 2.7.3 unstable. Here is how you can do it, but if you use it for production, you might want to wait for final stable release.
I upgraded my packages in my Ubuntu 10.04 Lucid Lynx Beta 1 installation and it required a restart. I did not realize that there was no space left on the root partition (I have two partitions, root / and /home). I restarted and was not able to login. I got a message saying something like