According to Martin Nordholts the single-window mode for future GIMP versions is almost complete except for some small bugs that have be fixed yet (= feature-complete). For instance, GIMP now saves when it is in single-window mode and starts the next time in the correct mode.

This feature is one of the most important changes for GIMP 2.8 which is currently scheduled for late 2011 or early 2012.

Also, there will be a new development version (GIMP 2.7.3) in the next days that will include the latest code updates.