Recently Felipe "JucaBlues" and I were hashing out some next steps for development, when I realized that Inkscape should NOT "support CMYK." Given that the Brazilian user community has really been pushing progress on the adoption of Open Source, including the use of Inkscape in print work, this might seem a bit surprising. 
However as we have been moving forward, implementing things, and trying to really support more and more professional use of Inkscape, the problem has become a bit clearer. What it seems to come down to is that we need to be sure to not get caught up in the low-level "how" of implementation of "four color CMYK" and instead change to focus on the higher-level "why" of "supporting prepress work." To get good, reliable, professional results in all forms of image work is a good thing, and people being able to get work done, and well, is key.
People who don't work towards print output are not often as familiar with CMYK and a CMYK workflow. What people are familiar with, at least when it comes to computer imagery, is RGB. GIMP, for example, has worked in RGB for the longest time (though it is now getting updated with new internals that will make working for print output much nicer). RGB images are split into three components: Red, Green, and Blue. Colors might be chosen with different modes (such as HSL), but are generally are stored in RGB. For photographs, web work, etc., RGB image work can be quite sufficient, and sometimes even preferable. There are also subtle variations such as sRGB, wide gamut RGB, Adobe RGB, etc. that can be used for specific needs.
Whereas RGB is creating colors by mixing three light primaries in additive color, CMYK is used with mixing four ink primaries in subtractive color. In general this is done by inverting the three RGB primary colors into inks and then adding a fourth ink that is plain black. This helps with getting true black, avoiding oversaturate paper, and other factors. Print artist often like working directly in CMYK so they can control sharpness of text, overprint of color, etc. However, the first big trap is that there is no such thing as "the CMYK colorspace." 
CMYK numbers/values are actually dependent on a specific device, and even two printers of the same make and model usually will give different results for the same input numbers. Given that an artist will choose to work in CMYK in order to more precisely control the output of a job, it is critical that color management is involved in the CMYK workflow to target specific jobs. Working on a job to be printed in a glossy magazine can be very different than one to be printed in a local newspaper.
SVG has been able to support CMYK via ICC profiles, and Inkscape has supported that in rough form since 0.46. Felipe and I have improved the interface for this more in 0.47, but there is still more to go. We also have been working with Scribus to help ensure color-managed SVG files become well supported. Some recent work for SVG 1.2 has added device-cmyk, but it turns out that this is for workflows that are not color-managed, and thus not really what end users need.
The other significant complication with a focus on "CMYK work" is that many professional jobs that are to go to print are not really "four color" at all. They might have four-color printing as part of the job, but then spot colors come in and easily expand to five-color, six-color and more. 
A company, for example, will often have a precise color they use for their logo, and this will be printed on its own plate in a pass of just that specific ink. UV coating, embossing and other effects might require their own color or 'channel'. 
Anyone who has ever heard "Pantone" mentioned has probably bumped up against this "not-just-four-colors" problem. And in addition to spot colors, modern printers have been expanded to use six or more colors, not just four, in order to get better results.
What that means is that to properly support end users needing to work on art that is going to be printed, Inkscape will need to support far more than only four colors. And more than just CMYK is a given (that spot color issue again). Artist will need to easily pick a target CMKY ICC profile for all jobs, have multi-color support preserved, handle spot colors and custom palettes, etc. So when it comes time to implement features, thinking only of CMYK will most likely lead to solutions that hurt, rather than help. So remember, think "professional prepress work" and not "CMYK work".
(Images 
This work is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License. Original apple image by Jan Mehlich)
