If you read through the FIM documentation for setting up PCNS, you will find that either FIM and PCNS need to be in the same forest or be part of a forest trust. From http://technet.microsoft.com/en-us/library/cc720594(v=ws.10).aspx:
Forest trusts are only required if PCNS and ILM 2007 are located in different forests. If this is the case, a forest-level trust must be established. This is required for Kerberos mutual authentication for the ILM 2007 server to accept the request from a remote forest host.
This can become extremely limiting, especially if both forests do not have the proper forest and domain levels. Theoretically, however, it’s possible to make Kerberos work over an External Trust (http://blogs.technet.com/b/activedirectoryua/archive/2010/08/04/conditions-for-kerberos-to-be-used-over-an-external-trust.aspx). With an External Trust, we don’t have any of the same forest functional level restrictions, opening up PCNS as a viable option for more customers. I would love to know if anyone has been able to get PCNS working in this configuration. In the mean time, I will have to set up a lab and try it out. I will report back on the results and let you know!