In order to create a trust between two domains, you need to have TCP port 445 (the Microsoft SMB port) open on both sides. Having open ports though (especially for SMB traffic) is an invitation to attacks by worms and other malware, so the few ports you need to keep open the better, right?
What's not often known though is that once the trust has been established between the two domains, port 445 can then be closed since the port only needs to be open during trust creation. See Domain and Forest Trust Tools and Settings for details.
Mitch Tulloch is lead author for the Windows Vista Resource Kit from Microsoft Press, which is THE book for IT pros who want to deploy, maintain and support Windows Vista in mid- and large-sized network environments. Mitch is also the author of Introducing Windows Server 2008, the first book from Microsoft Press about the exciting new server platform. For more information on these and other books written by Mitch, see www.mtit.com.