Exchange Server In Another Forest

Hello,

Here is the problem.  We are part of a larger organization which is composed of two separate Windows 2003 forests connected physically by a T1 connection.  I was recently informed that the other forest will be hosting an exchange server for our users.  The idea was that the GAL and the Calender Free/Busy would be centrally managed and a central mail server would be used.  My question is how to I set this up so that our users don't have to have accounts in each domain in each of the forests.  I would like everything to be transparent with a single sign on.  I could probably setup a forest to forest trust, but worry about exposure since this is a completly separate entity.  Anyone have some guidance on how this could be done.

Thanks...