In the new 8.10 release, "/DeviceCMYK setcolorspace showpage" produces a white page (as expected). "<< /ProcessColorModel /DeviceCMYK >> setpagedevice showpage" produces a black page.
What device are you using? I cannot reproduce this problem on pcxcmyk or display device.
using the pdfwrite device, the resultant pdf's (when viewed with ghostscript) are white & black respectively.
Patch http://www.ghostscript.com/pipermail/gs-cvs/2003-June/003395.html