Obtaining another company - sharing SMTP domain

I talked to Microsoft Advisory services about how to set this up.  I got all the nuts and bolts info about sharing the SMTP namespace, but they had no good answers for me about sharing a GAL.  Basically we'd want to be able to see/share the GAL on both sides.  So that users in each separate company could share the SMTP namespace, but also access the GAL for each company, making communication between the two companies easier.  Plus if we wanted to move a user from Company B into the Exchange org of Company A, there wouldn't be any problems with the GAL or contacts.  

Plus we'd like to know if there are any "Gotchas" with this type of scenario that we are not even aware of.  My boss is interesting in this setup for a test lab at first, so we do have some level of lab testing we can do, but he wants to simulate acquiring another company so if this happened in the future, we'd know just what to do.

Thanks

Is company A & B on the same domain? and are they 'normal' exchange (ie not SBS)?

No, both comanies are totally separate forests/domains/Exchange orgs...  They are either Exchange 2003/2007 standard and a mix of enterprise...  NO SBS...

Well the idea is how to deal with obtaining another company with an existing DNS/AD/Exchange configuration and how to absorbe that new company while sharing an SMTP namespace and being able to share GALs and communicate together with as little end user impact as possible.  So far the only real obstacle I can find is the GAL.  Our workaround was to use an expensive lifecycle management suite from Microsoft which is not a good option, or alternatively, creating manually a bunch of contacts for each company and sharing these contacts via public folder or PAB.  

Additional concerns are moving mailboxes back and forth between these two separate organizations.  As well as avoiding misconfigurations in Exchange that might cause NDRs and other potential mail issues.  

I should also mention that its possible that Company A will be Exchange 2003 only, and company B may be all Exchange 2007.  So we are looking for resources and advice that covers both of these Exchange platforms...

You could automate the GAL with a powershell script that could be run manually or via a schedule but moving mailboxes around, different domains, and i'm guessing over a WAN link, it going to get over complicated. Mailboxes are assigned to user identifiers which is found in the AD of the domain they are in.

I'd prob look at head office with exchange server/s and remote sites connecting in over RPC over HTTPS (outlook anywhere) and migrate other services slowly (DC at each office with DFS back to HO).

I know this is going to be a complicated configuration, just trying to figure out the best way to approach this acquisition scenario.  I also wonder how Microsoft handles this, if all their Exchange servers are in one master org contained in one master AD forest or if they employ some sort of trick to work around these issues.  

I also think public folders are going to be a problem.  If we move a mailbox from Exchange in Company A to Company B, the user will lose access to public folders in Company A since the Exchange orgs are separate and I don't see any way to replicate public folders across separate Exchange orgs...