We have had a few clients mention that after the installation with the STSADM command and deployment from the MOSS Central Admin the web part has an error adding to the site. We have found this solution works.
Modify the web.config from WSS_Minimal security to WSS_Medium security. At least a few of our web parts required Medium security and this is one of them.
The line you are looking for is:
Trust = "WSS_Minimal"
and you want to change it to..
Trust = "WSS_Medium"
-Chad
