Recently i was trying to do a quick export and import of some sites I had created from UAT to Live.
Using the STSADM was an easy way out, but i kept running into "Access Denied" errors, even though I was working on the Farm Web Front End Server.
After a quick google as to why, i discovered that;
1. For STSADM to work you need to run it with a local administrator account
2. This account needs to be a Site Collection Administrator on the target site.
After making sure these points were in effect.. The error disappeared.
See this STSADM Command Line Reference useful stuff :)