For all those still running BizTalk Server 2004, they should be aware that the Microsoft Mainstream support will end this year (http://support.microsoft.com/lifecycle/search/default.aspx?sort=PN&alpha=2004&Filter=FilterNO). 

One other point to remember about with the release later this year of BizTalk Server 2009 and according to all the existing documentation there is not a direct upgrade path from BizTalk Server 2004 to BizTalk Server 2009, you must first upgrade to either 2006 or 2006 R2.

Also for those still on BizTalk Server 2000 or BizTalk Server 2002 the migration to BizTalk Server 2009 is not supported, so you will also need to migrate to 2004, 2006 or 2006 R2 and then upgrade to 2009.