With the release of the Public Hotfix of October 9th (WSSv3 KB934525/MOSS2007 KB937832), StsAdm has gotten a new operation, the Mergedbs operation. A very cool operation, with which administrators are able to move sites from one content database to another!!
On his blog, Todd Klindt describes how to use the operation:
"On multiple occasions as a SharePoint administrator I have needed to move a Site Collection from one Content Database to another one. In the past this process was very painful and very manual. In this blog post I will show you how to move Site Collections between Content Databases with a single STSADM command using the "mergedbs" operation that was introduced in KB934525."