Example:
I was making all kinds of testing, to see if I could find any errors in the newest snapshot (13w16a), when i placed down some Mycelium, surrounded by dirt (clean dirt, No grass on all sides exept one) Then I left it there for about an hour or so, and when i checked up on it, it hadn't spread at all.

What I expected to happen was...:
When given time the Mycelium would spread to the surrounding dirt blocks.

What actually happened was...:
It didn't spread at all.

Steps to Reproduce:
1. Place down one or more Mycelium blocks, and surround them on the sides by dirt (as shown on the screenshot)
2. Leave it there for a while (30-50 mins maybe)
3. Come back and check up on it.