Setting up a untrusted cross-forest environment supports a simple Free/Busy query between two the forests using „galsynced“ contacts. This feature I described in article Cross-Forest Free/Busy without Federation and its corresponding troubleshooting guide.
In this article I want to share some experiences with troubleshooting the Cross-Forest Delegation feature. Cross-Forest Delegation allows completly to manage a delegated calendar even if people are placed in different mail-organizations. But there must be a domain-trust in place.
To synchronize objects I prefer NETsec's GALsync, which is much easier to use than Microsofts FIM etc.