Today's tip is brought to you via the BizTalk 2006 Deep Dive training course that I'm on, run by Alan Smith of the Bloggers' Guide to BizTalk fame..
In BizTalk 2004, if you wanted to create an empty output element in your target document via the mapper, you had a couple of options. You could either use a string concat functoid with one empty input parameter, or use a value mapper with 'true' and an empty string as its inputs. Bit tacky really, and in the case of a string concat also a bit slow as it uses inline C#.
Well rejoice, for in BizTalk 2006 there's a new drop-down option on the "Value" field of the mapper that lets you pick "".
Lovely! Especially handy if you are using the little-known but amazingly handy Property Demotion technique to populate message fields on the way out.
Incidentally, further to the system context properties described in the link above, you can also demote your own promoted properties from your own custom schema back into your message in the send pipeline.