do you have a firewall and are you natting any addresses to the your exchange server?

This is problem in clustering.  I did not realize it was fixed in Service Pack 1.  The way to fix this issue pre-SP1 is by going into the Windows Registry and disabling DHCP Media Sense.

 

The MSKB article can be found at http://support.microsoft.com/kb/q239924/.

 

1.Use Registry Editor (Regedt32.exe) to view the following key in the registry: Add the following registry value: Description: This parameter controls DHCP Media Sense behavior. If you set this value data to 1, DHCP, and even non-DHCP, clients ignore Media Sense events from the interface. By default, Media Sense events trigger the DHCP client to take an action, such as attempting to obtain a lease (when a connect event occurs), or invalidating the interface and routes (when a disconnect event occurs).2.Restart your computer.