I’m creating some business cards and my print shop needs PDF format output with a CMYK colour profile. Neither of these can be done with the Gimp as installed, so this is what I had to do in Ubuntu Oneiric Ocelot with Gimp 2.6.11 to finish the task:

1) install gimp-plugin-registry and icc-profiles from the apt repository (“sudo apt-get install gimp-plugin-registry icc-profiles” or use the software centre);
2) open the image I want to send to the print shop and select “Image/Flatten Image” (if the image has multiple layers) and then “Image/Separate/Separate”;
3) in the dialog, accept the default options, but optionally check “Make CMYK pseudo-composite” (this doesn’t seem to affect the final product, but it does generate an image with the colour layers blended in a way that is recognizable — that way you can see that you are getting what you expect);
4) when the new separated version of the image pops up in a new window, select “Image/Separate/Export…” and save as a TIF file;
5) use “tiff2pdf filename.tif -o filename.pdf” to create the pdf file for the printer.

The most useful info I found on this topic was here: https://wiki.archlinux.org/index.php/CMYK_support_in_The_GIMP and http://ian-pullen.suite101.com/how-to-save-cmyk-images-in-gimp-a107979.