Since the ICC colour handling, pdfwrite no longer sees images in their original colour space, but instead sees all images as being in an ICCBased space. The conversion of CMYK images to RGB depends on knowing that the image was originally in CMYK and so no longer works. This may also have implications for the ColorConversionStrategy control.
Fixed in revision 11755, patch here: http://ghostscript.com/pipermail/gs-cvs/2010-October/011782.html