(Richard Jackson kindly provided the information for this article).
To print with ICC profiles using the GIMP:
Go to Edit | Preferences | color management and set as follows....
Mode of operation : Print Simulation Monitor Profile : Put your monitor profile here Display Rendering Intent : Relative Colorimetric Print Simulation File : Your new ICC custom profile Softproofing intent : Relative Colorimetric Check the Mark out of Gamut color checkbox - I make it bright red so it's obvious when I'm not looking at an actual softproof - see later. File Open Behavior : Keep embedded profile.
You must also set the display filters to off. This is under View | Display Filters... and make sure there are no active filters. You'll know if there are active filters because the 'out of color gamut' color that was chosen under preferences will be all over your image.
Also, I needed to tell windows to use the new ICC file for the printer since Gimp does not actually have any ability to select the actual printer profile - it only uses it for softproofing.
(This is why you must specify Relative Colorimetric as the rendering intent when soft proofing as the Windows ICM engine only uses RC when converting to printer profiles).
(If you are using a Mac, you will need to do the equivalent in Colorsync, which is detailed in this article).