What you've been asked to do seems reasonable and possible.  Your test setup could be on the same network as production as long as it is not some kind of clone (renamed or otherwise) of the production Forest/Domain. I can think of a few ways to set this up, but it all depends on your infrastructure. You could keep the test domain isolated via firewall, etc. and open up RDP port (3389) for access. You could setup a one-way Domain trust with the production so that users in the production Domain could be granted access to resources in the test domain. Those are just a couple that came to mind - It really depends on your infrastructure, security requirements, remote access setup, etc.

If you decide to setup a trust relationship, here's a link to more information on that:
Active Directory Domains and Trusts
http://technet.microsoft.com/en-us/library/cc770299.aspx

Even though you shouldn't need to go this far, similar situations arise during mergers and acquisitions,

Restructuring Active Directory Domains Between Forests
http://technet.microsoft.com/en-us/library/cc786927(v=ws.10).aspx
The link is for 2003, but the process would be similar

Hope that helps...
-JB

Thanks for the insight!