If you use the Open With External Editor command, and have your preferences set to use PSD format for doing so, and then convert the image to CMYK in Photoshop, it will appear as a section of misaligned separation layers when it returns to Aperture. You will not get an "unsupported format" message as may occur in some other situations. For a longer answer, see Question 5 of Aperture: Avoiding issues with your External Editor (FAQ).
This document will be updated as more information becomes available.
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