Along with mergecontentdbs another operation has been recently added to STSADM, renamesite. KB939535 gives some guidance on how to use this new operation. At first glance, I assumed it worked like renameweb, which was nice as there is no way to rename site collections without locking them, backing them up, deleting them and restoring them with the new name. After I read the KB I was disappointed. It seems that renamesite does not work with path-based site collections. A path-based site collection is in the form of http://server/sites/sitename. 99% of the installations I see are in the form. Instead, renamesite only works with host-based site collections. Host-based site collections are site collections that are created using host header mode. Host header mode allows multiple site collections in the same web application, but all have top level names like http://host1 and http://host2. If you're using host header mode, you can use renamesite to rename them. If you're using path-based site collections like most of the SharePoint using world, then this operation is useless to you.