Here is a small issue new to me, when you set up trust between forests and define it as a transitive one, users can log on as expected from sub-domains if you use the UPN or domainuser syntax.
This is quite as expected, but on older Windows versions, users are presented with a dropdown list showing the domains available to use at logon. This list will not show the subdomains of the forest trust.
In order to see the subdomain you will have to create shortcut trusts for every subdomain.
More information about shortcut trusts can be found here: http://technet.microsoft.com/en-us/library/cc737939.aspx