Well I have been doing this for a while now (Using Enterprise Library Logging and Exception Handling with BizTalk 2004)
and I happened to google it (how did this turn into a verb?) one fine day and found this great blog by Andy Morrison on the topic.  

Remember to strong name your Enterprise Library assemblies before starting this.  Here's an article by Tom Hollander about strong-naming your Enterprise Library assemblies.

I remember when Enterprise Library first came out and I was thinking of using it with BizTalk 2004 I thought that it would be a huge challenge considering BizTalk is not your run of the mill application.  After about 5-10 minutes figuring out what
configuration files to use in BizTalk (<installation folder>\Microsoft BizTalk Server 2004\BTSNTSvc.exe.config)
I was up and running with the Logging Block.  Happy Logging.

        -B