I found a great online tool tonight, the online Lighting Diagram Creator.  Check it out if you need to create such diagrams for yourself.

It has a good range of objects to drag and drop and each can be rotated to fit. The basic version then lets you export the image in a number of formats allowing the setup to be uploaded to blogs or kept in the image archive along with the photographs.

UPDATE: If you want to create these things on the move, would rather not lug a laptop and have an iPhone check out Strobox on the Appstore.  It could do with more features as there is a save to image option but nothing to let you edit a layout once saved. Still, useful for a free app.