Saving a color ID CS5 file as a grayscale printable PDF?

There isn't such an option in InDesign, but you can do it in Acrobat Pro.

In Acrobat Pro 9 or X, open up the Preflight panel. (In Acrobat 9, choose Advanced > Preflight. In Acrobat X, choose Edit > Preflight.) Click the Single Fixups button at the top. Use the Search panel to search for "grayscale". Run the fixup "Convert color to grayscale"

You can't, you have to do it in Acrobat afterwards.  In Acrobat 8 you go Advanced > Preflight > PDF Fixups > Convert to Greyscale.

You can't EXPORT to Grayscale PDF, but you can Print to it...

IF you decide to try that, it's imperative to go to the preference for Appearance of Black and set it to Output All Blacks as Rich Black to avoid screened type.

That's probably because you're on a Mac in Snow Leopard (10.6.x) or later, and its been replaced:

Acrobat's Adobe PDF Printer Replaced in Snow Leopard | InDesignSecrets

But I think Peter meant: File > Print > Output > Composite Gray

With all the changes in workflow required, I'd just use the Acrobat method.

Steve is correct on both counts. If you're a Windows user, printing to PDF is faster becasue it happens in one step, but for Mac folk you have to jump through a lot of hoops these days to print to .ps file, then distill as a separate step. The only reason for even attempting on the Mac side would be to see if there is any significant difference in the gray values between the two applications when they do the conversions.

Which brings up the biggest trap in doing any sort of automated conversion -- bad rendering. Colors of wildly different hue can easily end up looking like the same gray value if they share the same luminosity. I think it's always better to do individually adjusted grayscale conversions of each image and make a "real" grayscale file.

There's an alternate method if you want to play with the grayscale values. In Acrobat 9 Pro or Acrobat X Pro, open the Convert Colors tool. This is in the Print Production toolbar in Acrobat 9 (Advanced menu) or Tools > Print Production in Acrobat X. Use the default conversion (Any Object, Any Colorspace, Convert to Profile). In the Conversion Attributes section of the huge dialog, choose a grayscale profile (either Gray Gamma or Dot Gain options).

Where do you get to play with the values (beyond closing the file without saving and picking a different profile on a new attempt) using that method? It's document-wide, not image specific, and I don't see any preview. It's probably fine for quick and dirty conversion and the results are probably comparable, or even perhaps identical to printing to grayscale using the same output profile, but there's no real user control.

You get to try out multiple profiles. And of course there's no preview. If you had a project where you had to do this regularly, you could create some test PDF files and see which worked the best for you.

If you're working with document with critical images, I'd use Photoshop tools (probably a Black and White adjustment layer) which would give the best results.

I think we're in agreement.

I think we are too.