I just got bit by this one a second time. So it is time to store the information here to avoid a third occurrence.
If you get an exception that the functoid was not found.
For example:- 'Exception Caught: Functoid not found:
guid({5DE500CC-45BC-454b-A23D-24449899042C}) funcid(6123)'
Check that the dll is installed in the mapper extension folder 'C:\Program Files\Microsoft BizTalk Server 2006\Developer Tools\Mapper Extensions' as well as the GAC.
Geordie