If they are in the same forest, yes you can. Take a look at the following link:
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_callmg/4_2/srnd4_2/uc4_2/42drctry.htm#wp1067012
There are definitely some added complexities and considerations in this sceario. Take a look at the following note from the previous link:
"In a multiple-domain AD forest, try to keep the users for a specific Cisco Unified CallManager cluster within a single domain, and follow the guidelines described previously. If a single domain is not possible because users are spread across multiple domains, set the User Search Base to the lowest point in the tree containing all domains with users serviced by the Cisco Unified CallManager cluster. In structures in which serviced child domains are under the top-level domain, the User Search Base must be set at the root of the entire AD forest. In all cases, though, try to ensure that a domain controller for each serviced domain is collocated with Cisco Unified CallManager, or that the network is sufficiently resilient and fast to allow remote searches with no greater performance degradation than occurs with local searches."
Hope this helps. If so, please rate the post.
Brandon
