When backing up a site collection and restoring it in the same SharePoint 2007 (MOSS) web application, I received the following error message:

 No content databases are available for this operation. Create a content database
, and then try the operation again. To create a content database, click "Content
 databases" on the Application Management page, select the Web application to us
e, and then click "Add a content database".

However I had a Content database which was in the Ready state and allowing new sites...

Luckily thanks to Akila's SharePoint blog article I realised this is because the GUID of the Site Collection already exists in this Content database - therefore the Restored Site Collection needs to be in its own Conent database - creating another content database made my restore work.