If you apply SP2 or greater to the client you have SSMS 2005 tools loaded on you should be able to connect to 2008. I am trying to find a Microsoft article to confirm it.
http://support.microsoft.com/KB/946127
This article also shows you some of the problems you may run across using it. If you have SQL Server 2008 installed and licensed you can go ahead and install SSMS 2008 it is backward compatible to connect and manage your SQL Server 2005 instances.
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