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