In CS3 the easiest way to achieve this is by using "Print" to PDF rather than "Export" to PDF. In the print dialog ("Setup" button on Windows, not sure about Mac), select Adobe PDF as the printer, open the standard print dialog for the printer, and select B/W printout.
You can use the same feature as desicibed above in MAC.. you need install adobe pdf write or pdf writer in MAC OS then select B&W setting to create the pdf.
Alternately you can use any pdf tool open in the pdf file with Acrobat 8 and use the convert the pdf to B&W from inbuilt feature
