http://geekswithblogs.net/sureshg/archive/2006/02/14/69456.aspx
http://support.microsoft.com/kb/899191/
Says this is because your invoker, Biztalk is not authenticating with the DTC participant(in this case the db server), it also contains a way to fix this as modifying the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSDTC
Value name: AllowOnlySecureRpcCalls
Value type: REG_DWORD
Value data: 1
value data to 0, although setting this value to 0 could have security implications that are beyond the scope of this blog :DD..just kidding.