Something strange; I have a grayscale pdf file, which I import in Indesign CS5.5 (mac), the pdf gets changed to CMYK, even after exporting to pdf the colors stay CMYK.
This is very unwanted, it does not happen every time, is there perhaps something I can do to prevent this from happening?
Sounds like it isn't really grayscale in the PDF, but a CMYK file with only a K component, and the profile is different from your ID CMYK working space. I get this from time to time as well. I solve it by using Acrobat's Convert Colors to change the output profile.