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My company has two domains, I want to merge them?

Ok, a company I manage IT for has 2 domains, all Windows shop, in two separate locations. One is in Washington state, where all the users are. In the office there are 1 domain controller for user's desktop logins and Active Directory, 1 backup, several in house application servers, file servers, and nightly backups run to a file server.

In CA, in a data center, resides the other domain, which the user's Exchange and OWA accounts run off of. Also has a main domain controller and a backup. This domain location has the web server in a DMZ, and also a Citrix connection from WA to a server here in CA.

The two domain names are different. How would I go about merging them? I don't know where to begin. The usernames are different for the same users on the domains. What would you suggest?

Some things to consider are many of the user's email/Exchange account names are different from their desktop login. I'd like to keep it this way after the domain merger. Both domains have different ISP's and firewalls of course.

On a side note, I also plan to upgrade the domain controllers to Windows Server 2008 from 2003. Only the CA side will need to purchase a physical server capable enough to run 2008.
1 Solution
I would take the opportunity to look at hosted exchange as this is the only part in the datacenter... If that's not possible.

1 setup vpn between the sites.
build up the new hardware and install exchange in ca
Create mailboxes on the new exchange Change over the receiving server to let email flow in to the new server.

Export all the mailboxes to pst using exmerge or powershell.
Import the mail to the new system using same tools.

As the users already exist in both domains you can't port the over. You have to extend your main domain to also host exchange and migrate the data over.  
I don't normally refer to blogs as references but this one seems to be exactly what you are looking for.

High level for you to start planning/researching:

Create a Two-way Forest Trust between the domains
Choose which domain shall become the de facto or create a new domain
Use ADMT tool to migrate the objects from the deprecated domain(s)
Use Powershell and/or Exmerge to migrate the mailboxes
Update recipient policies in Exchange
Update external domain MX records
stadminAuthor Commented:
It appears Active Directory Migration Tool will be what I need. What can you guys tell me about how Exchange will be merged into the one Directory? It is currently on the domain that will be moved over to the other one. Will I be able to run redundancy, meaning having the primary domain controller in WA and the secondary in CA, and the Exchange server in CA?
Basically exchange doesn't have to be in any location it should be close to the biggest user group. also for redundancy if possible I would create a secondary exchange in another location especially if you are going to upgrade to ex2010 .

First start with how you want to end up and then plan migration..

IE you want to end up with 1 domain 2008 with exchange 2010 both high available.
2 sites x users per site
X member servers

Needed is at least
1 domain controller per site (optionally 1 extra in the main exchange site for global catalog etc )
1 exchange server per site in dag together basically to let the mailstores be redundant
At least 1 cas role server and at least 1 hub transport  server (depending on number of users some of these  
Roles can be combined.

oNce you have this you can check the existing hardware.

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