I've planted a mushroom farm before and got it to grow small mushrooms. On the previous world I had created I actually got the tall mushrooms to grown in the farm -- they looked very cool. I started a new world the other day on the 1.1 patch and I've noticed that -- even though I did the same things -- there were no large mushrooms. I suspect the depth is wrong, even though glowing mushrooms are growing.
Here are the things that I think you need to do:
- heat source (this seemed to help the tall mushrooms on the last world, they were more likely to grow near a torch)
- depth (think anything under level is fine, but does the depth matter for tall mushrooms?)
- time (wait, wait, wait)
- height (I think at least 20 blocks of space)