Robert McKee's Story Seminar - 19-21 April, 2008 http://www.mckeestory.com/
I took this course last year and learnt more from it than all the other courses I did put together. There are mechanics and rules in writing and of course you can bend them but you need to at least know them to bend them. It gave my writing more levels and its no coincidence that I have had more success since.
Its based around film rather than sitcom but he covers the genres and shows how easy it is to adapt what he says to all writing, even novel. Most of all he helped me develop natural and original story.
Just thought I'd pass it on as someone here recommended his and Steven King's book on writing as the best you can get. I have McKee's and now King's.