You could add it as a custom field using WMI query - however we have had a number of requests for this and have added it to systemhound 2009 which will be out shortly.
The WQL query would be:
SELECT domain FROM Win32_ComputerSystem
How to configure systemhound:
http://www.systemhound.com/wiki/index.php/Central_Configuration_systemhound#Capturing_custom_information_using_WMI