If your domain is registered with Google (which means your DNS is pointing to Google) and you receive "Another blog or Google Site is already using this address" error when you tried to add your own domain name to Blogspot, do the following
1. Log in to your Google Apps at www.google.com/a/yourdomain
2. Under Dashboard > Service Settings, see if Sites is listed. If it is, skip to step 4.
3. If Sites is not listed, click on "Add More Services" and click "Add It Now" for Sites.
4. Click on Settings > Sites > Web Address Mapping and remove any mappings there.
5. Under General > Uninstall Service, click on Uninstall Sites.
You can now add your own domain in Blogspot.