Praise aside, it is a good article on how subnet and site definitions are used to find a Domain Controller closest to you, and especially how it works across forest trusts.
Update: There’s a part two which I came across later. This covers a specific edge case.
Using Catch-All subnets in AD – Wanted to know how catch-all subnets in AD Sites will interact with specific ones. This one explained it. The specific one takes precedence. Which is exactly what you want. :)
Do cross forest trusts need PDCs that can communicate with each other? The answer seems to be NO, check out this discussion for more.