Wow I was about to post with the exact same question. Let me elaborate a little on my situation. Don't mean to hijack your thread, just trying to avoid duplication.
Two separate Exchange organizations running on two separate domains in two separate forests. There are currently no trust relationships whatsoever. These two organizations are operated by subsidiaries of the same company. It would be nice if users from each subsidiary could view the GAL of the other company, or some other query-based address list even.
Any help would be greatly appreciated!