I was responding to a thread (http://forums.adobe.com/thread/1165011?tstart=30) yesterday about black turning to 4-color in Acrobat and realized that (unless I'm missing something) the Output Preview (and I assume print output) has change in Acobat XI. Something tells me we might be seeing more questions about changing black values.
In Acrobat X the Output Preview simulation profile always showed the CMYK values of a PDF/X-1a or an unprofiled PDF as unchanged—the appearance of color changed not the numbers because the numbers would always be output unchanged.
In Acrobat XI the Output Preview is working differently. For PDF/X-1a the numbers change for any profile that conflicts with the Output Intent. For untagged PDFs the numbers change for profiles conflicting with the current Acrobat working CMYK space.
I think this means PDFs exported without document CMYK profiles (i.e. the Press Quality or High Quality Print presets) might have color problems because the host Acrobat working CMYK space could be anything and there is a chance of random CMYK to CMYK conversions.
Here's a PDF/X-1a with Europe ISO Coated as the Output Intent in Acrobat X:
Changing the simulation profile changes the preview:
Here's the same PDF opened in Acrobat XI, with the US Newsprint simulation profile the output numbers change rather than the preview:
