Service Account: I created two service accounts, one on each machine. The service accounts had the same name. When I went to link the resource account to the account in the user forest, accessing the linked domain controller failed.
Lesson learned - The link failed because I didn't qualify the service account with the user forest (e.g. userforest\mySvcAccount). So Exchange tried to link to the user forest domain controller with the resource service account and failed. Had I created service accounts with two different names, this problem would not have tripped me up.
After I fixed the service account issue, the last step of in the "linked mailbox" wizard failed with an invalid address. After much head-scratching, I discovered the system attendent had failed with an MSExchangeSA event ID 1005: "Unexpected error The Local Security Authority cannot be contacted ID no: 80090304 Microsoft Exchange System Attendant occurred. "
Dave Goldman blogged about this: Creating a new mailbox in Exchange 2007 with the new-mailbox cmdlet fails with Address List Service not Available, but only addressed one aspect. My problem was choice (4) - SA stopped. I haven't figured out why it's failing, but restarting the SA allowed me to complete the mailbox link.
OK--everything is set. Next time I'll report on the results of impersonation.