"The FTP site cannot be started. Another FTP site may be using the same port."
This occurs when right clicking the FTP site and trying to start it.
Now I know that I do not have any services listening on TCP21 on my server. A netstat shows this.
I did however have a test service running on TCP21 for a small period of time to verify the firewall portforwarding rules are setup correctly.
When attempting to start a FTP site on IIS7.5, it does not check that TCP21 is free, it checks prior at some stage. To get it to recheck properally you need to restart the "Microsoft FTP Service"
Ahh now all is good... this bug had me scratching my head for a second!