I've just found and used this: Online Sprite Generator and wanted to make a note of it before I lost it and could never find it again.
It's very easy to use. Take all your icons (or any images) that you have on your site and put them into a single zip. Upload them to the site and generate the CSS and single image file. You can download the single image file and copy paste the CSS that's generated for you. It took me all of 10 minutes to do a global find and replace for the CSS names and it worked immediately. I left all the options on the generator web site at their defaults.