I'm sure I've written about this; at the least, it's in my install documentation and also the Official Microsoft BizTalk Installation documentation.
When firing BizTalk up pointing to a remote SQL Server, you might see the following error: "The transaction manager is not available".
I'm careful to point out you must set client network access for DTC in my installation documentation. I also found reference to a MS article titled “How to enable network DTC access in Windows Server 2003“. Be sure to restart DTC when you've finished the configuration steps.
I located a link to “How To Use DTCTester Tool“, which contains a DTC Test application and instructions on how to set it up to test your DTC installation is configured correctly.
Windows Server 2003 SP1 invited the Windows Firewall into the mix, so configuring network DTC is all the more important on both the BizTalk and MSSQL servers. I have a few details which I'll provide later, but here are two MSDN articles that helped me:
How to adjust the MSDTC settings:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/deploying/htm/ebiz_depl_assemblies_tfrh.asp?frame=true
MSDTC Changes in Windows 2003 SP1:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cossdk/html/2627a956-60b3-4d26-bc04-e0676ec97786.asp