Good afternoon everyone!  I wanted to post a clarification and a longer answer to a question that I put on my blog a few days ago following a webcast that I did on Exchange and PowerShell scripting here is the Q/A post:
Introduction to Windows PowerShell Scripting in Exchange Server 2007 (07-11-2007)

Evan Dodds a Exchange management guru in the product group connected with me and added some great feedback around my question and answer.

Here was my original Q/A:

Question: On a mixed E2K3/E2K7 environment, can the shell be used to manage both sides?
Answer: Yes it can, however it will only allow you to access the objects that are Exchange 2003 specific.

One of things that I want to make very clear is that the exchange management shell (EMS) can manage both Exchange 2003 and Exchange 2007.  However, EMS was designed for Exchange 2007, in that it can be used to modify all Exchange 2007 objects and attributes but only a limited set of the Exchange 2003 attributes (although all objects can be accessed if they’re visible to the Exchange 2007 EMS).

Evan made a great point in that in mentioning that there is a lot of subtly to Exchange 2003 and Exchange 2007 management for interoperability and configuration.  The story gets even more trickier, when you are trying to mange Exchange 2007 from the Exchange 2003 side. So it was important to me after talking with Evan to elaborate on my answer.  Take a look at the resources below to get even more detail on managing a mixed Exchange environment.

Additional Resources:

Evan made a great post on management in a mixed Exchange 2003/Exchange 2007 environment on the Exchange Team's blog:
Exchange Server 2007 and 2000/2003 systems management co-existence  

Excellent TechNet article: Managing Exchange 2003 Settings in a Coexistence Environment

Check out Evan's blog: Evan Dodds - Microsoft Exchange Server Blog 

He has a lot of fantastic information around Exchange and management.