Yep, there are already a lot of ways to convert a color image to grayscale in Photoshop or Camera Raw. But all of them except this technique mean your image is converted as a global correction. Using color channels as grayscale layers means you can use layer masks to alter the grayscale conversion and allows you to optimize for the best possible result. The technique allows shows how to re-convert your grayscale image back to a color image for the purposes of color toning.
Please note this video is in Quicktime format. A player can be downloaded from apple.com.
This movie extract can be found on the DVD disc that comes with Adobe Photoshop CS4 for Photographers: The Ultimate Workshop, jointly authored by Martin Evening and Jeff Schewe. The book is published by Focal Press and contains 400 pages of tutorials and tips on how to achieve the best results out of Camera Raw and Photoshop. You can also find out more about the book by visiting the Photoshop for Photographers website where you can preview more of the content that will be appearing in this book.