A great design, if the topology can handle it, would be to allow the
Exchange server to be more or less self-contained.  By that, I mean that you
would place the Exchange servers and resource forest DCs in the same central
site.

If you de-centralize the Exchange forest, you'll very much want to put GC's
for that forest in the same site as the Exchange server because Exchange
heavily relies on the GC's. In fact, two minimum is often a best practice
for each site and more if needed. That's for server usage but the clients
would as well.

As a consultant, I would be remiss if I didn't suggest that you find a
consultant to help you out during the desing phase at least.  If you're also
a Microsoft PSS customer, you may want to consider a supportability review
after plan formulation to verify the design and maybe fine-tune it.  Can be
extremely helpful.

Drop a note off-line if I can be of further help and if you haven't already,
have a look at the design and deployment docs at
http://www.microsoft.com/exchange/library as well as the multi-forest
considerations documents also located on Microsoft's web site (I prefer a
google search like this:
http://www.google.com/search?hl=en&q=multi-forest+considerations+site%3Amicrosoft.com )