I spoke at the Charlotte SQL Server User Group last night.  Will Sweeny confirmed that MSFT will continue to offer the free trade-out to 64-bit licensing from 32-bit.  In other words, if you upgraded your 32-bit SQL 2000 to 32-bit SQL 2005 and are considering a new 64-bit server, you can simply swap the 32-bit license for the 64-bit license.  Of course, you will need to de-commission the 32-bit server.  In SQL 2000, this is available, but the 64-bit licensing is only for Itanium based systems as there is no x64 flavor for SQL 2000.