Contributed By: Cliff Hobbs [MVP ConfigMgr]
Yes a SMS 2003 Client CAN be assigned to a ConfigMgr 2007 Site. Just bear in mind that although the SMS Client will continue to function as normal it won't be able to take advantage of any of the new ConfigMgr features (as it's not aware of them).

However, the same is not true for a ConfigMgr Client being assigned to a SMS 2003 Site. This is not supported and if you do manage to do this you'll find the ConfigMgr Client doesn't function properly.

More information at:

http://technet.microsoft.com/en-us/library/dd547071.aspx