As a workaround, try to log in as a local user that does not have valid credentials on the SMB server. The username and password prompt should appear. Specifying a username as part of the SMB URL does not override the local credentials if they are valid for the SMB server.
Not prompted for username and password when mounting SMB shares
When trying to mount an SMB share, OS X Lion will try local credentials to mount an SMB share. If the local username and password are the same as the username and password used to mount an SMB share, the share will mount without prompting for credentials.