What is the conversion factor between fresh and dry herbs/spices?

And when to add to your cooking makes a difference:
Add dried at the beginning of cooking, and fresh towards the end of cooking

I'm an avid home cook and I use the 3 to 1 ratio also.

I'm going with Lexmccall above: as a cookbook editor, I always use the 1 dry to 3 fresh ratio as well.

3 fresh to 1 dry. e.g. if the recipe calls for 1 Tbsp of fresh thyme, substitute 1 tsp dried thyme. (convenient, I think, that 1 Tbsp=3 tsp)