ADFS on a Domain Controller

This is more of a design question and I am not sure if I will go with Windows 2003 r2 or 2008.  We have several Domain Controllers and we want to start playing around with ADFS.  I am going to build a seperate Federated Services Proxy server in our DMZ, but it is alright to installed Federated services on a domain controller?  Every slide or video always shows a seperate internal federated services server, but everywhere I have always worked(not massive companies) we have always been fine installing additional services on the DCs like DHCP and DNS.  If anyone has and good experience with putting ADFS on a DC let me know as we already have about 9 DCs for 6 sites any only 3000 users and the federated authentication would not even be used that often.

Thanks,

Dan

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Dan Heim