Microsoft SharePoint Team Blog : Possible Issue with mergecontentdb STSADM Operation
This one is another doosey… Be careful with mergecontentdb when you are using STSADM in WSS and MOSS.
Large databases can cause this command to seem to be not working when it can take a long time to complete. Canceling this once it is begun can lead to corrupted databases.
Microsoft SharePoint Team Blog : Possible Issue with mergecontentdb STSADM Operation