I’m sitting in one of the coolest talks at PDC. ChrisAn & GioDL are showing how you can use the nascent "Oslo" language technologies to write your own textual language. MGrammar has been described as yacc on crack.

You should check it out (probably starting tomorrow) at: http://channel9.msdn.com/pdc2008/TL31/

UPDATE: You can download the SDK here and start playing with writing your own custom language. Fun stuff!