Convert Color Images to Grayscale
Rorohiko ("Lightning Brain") recently released a free beta of an InDesign plug-in that converts color bitmapped images (such as TIFF, JPEG, and PSD) to grayscale. Actually, more accurately, you have to install three plug-ins to make it work, but it's easy to do (just download a file from their Web site and then drag the plug-ins into the Plug-ins folder inside your Adobe InDesign folder).
But it's not perfect magic: For example, transparency in an image apparently is not supported in this version, so pictures become opaque. Also, as the folks at Rorohiko are quick to point out, this plug-in offers no controls for how an image is converted (there are lots of ways to convert color images to grayscale; see Real World Photoshop).
I haven't tested this plug-in extensively, but it does appear to work quickly and all results end up on the black plate of color separations. No word on whether this plug-in will remain free or will cost money when it's finished.
Rorohiko also makes a bunch of other free (and commercial) plug-ins, including one that exports all your document text into a single file, one that makes an InDesign library out of a folder full of images, and (one of my favorites) something called ChatterGoofy, which is a replacement for QuarkXPress's Jabberwocky XTension.