Organisationh Unit field also called "department" field.
When you are installing your Microsoft CA server, you need fill out all the blanks, one of those blanks is "department", you need fill that out as well.
For example "sydneyvpn", then the OU will be "sydneyvpn".
You need to reinstall the Microsoft CA server to re fill out all the blanks.
Otherwise, all the routers and PIX using SCEP to enroll, there is no way to get a OU field. (Because you can not specify the OU during the enrollment.)
For VPN client 3.x and VPN 3000, we can fill out a form to enroll to Microsoft CA server, we can manually put a department or OU name there.
So for VPN client 3.x and VPN 3000, we do not have this problem.