Well these are the answers taken directly from Microsoft's homepage, this includes the paragraph about every tester see the following page: http://www.microsoft.com/sqlserver/2008/en/us/licensing-faq.aspx
Q. What is SQL Server 2008 Developer edition and how is it licensed?
SQL Server 2008 Developer edition is SQL Server 2008 Enterprise licensed per developer and must be used for designing, developing, and testing purposes only. There is no limit on the number of installations as long as those instances are used only by the licensed developer/tester.