Quite a stretch for clustering in 2008

Microsoft Clustering Services (MSCS) have existed in one form or another since NT4, but have always suffered from a significant limitation.  All cluster resources had to exist within the same logical subnet, or else you couldn't create the cluster itself.  Windows Server 2008 allows for some flexibility in that regard, with the ability to create nodes of a contiguous cluster in different logical subnets.

Before we dive too far into that, you may want to see the official MSFT information here:

http://technet.microsoft.com/en-us/library/cc770625.aspx

So what does this mean for you and I?  It means that we can create CCR clusters on Exchange 2007 that stretch between physical locations and subnets.  However, to do this you'll need to be on Server 2008, the function just isn't available in Server 2003. This allows you to provide basic availability for Mailbox Role (MBX) servers in your Exchange environment, but doesn't take care of everything when it comes to DR planning.

First off, this applies only to Exchange 2007 Enterprise Edition, and then only to MBX role servers.  While most other roles are natively fault-tolerant with multiple servers installed with the same role able to stand in for each other, organizational or regulatory rules might not make that kind of redundancy possible.  Edge servers are the biggest example of this.  Since they're not tied into the domain structures, they don't contain any way to quickly flip traffic from one Edge server to another in different sites.  Third-party tools (see disclaimer below) can often take care of that function for you, as can working with your DNS provider to facilitate moving the MX records in the event of an emergency.

If you have legacy Exchange 2000/2003 servers, you're also not able to take advantage of this new MSCS feature set on those boxes.  The same goes with any non-Exchange tools, like SQL, anti-virus servers, anti-spam servers, etc.  Even if these servers run on Server 2008 clusters, they'll require some third-party intervention to handle the data replication for those systems.  This would include things like Blackberry servers, GoodLink systems and other non-Exchange remote email tools.

Finally, keep in mind that CCR clusters can only extend to Active/Passive, 2-node configurations. That will mean you can't use these solutions if you need to go beyond that model - which Exchange 2007 easily allows without CCR involved.

Server 2008 Failover Clustering is a great method for basic High Availability for Exchange 2007 CCR systems - even across subnets.  With some additional tools, it can become the center of an Exchange DR solution set that can help your organization withstand even site-wide emergencies.