Tried a yeast substitute in a bread recipe the other day. It worked -- about 50%. I used some baking soda and lemon juice in place of a yeast packet. It was the recipe for Tom's Bread, commonly found on the web and in this blog. The bread raised about half of its intended height. Still, it worked and I ended up with a loaf of bread that I could slice for my kids. They loved it. I plan to keep tinkering with yeast subs to see if I can get more raising out of them.