Gimp 2.8 has been released and it includes the requested Single Window Mode feature:
http://www.gimp.org/release-notes/gimp-2.8.html
To install gimp 2.8 on ubuntu run:
sudo add-apt-repository ppa:otto-kesselgulasch/gimp
or if that fails (due to proxy) then try adding the deb source into Update Manager sources list manually:
deb http://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu precise main
Then:
sudo apt-get update
sudo apt-get install gimp
Then to enable single window mode, open an existing image or create a new one.
Then go to Windows menu -> Single Window Mode