Quote from btwsupporter
The only blocks I haven't seen them spread to are Glass, Chests, Workbenches, Furnaces, Etc. even when put in lowest light level for 3 days IRL Although everything around it was covered in mushrooms.
Well I don't think glass, leaves and all the "transparent" blocks allow mushrooms on them, but in SMP I've gotten a mushroom on a workbench.
Also, in Etho's Lets Play in one of the episodes (somewhere between 85-95) he went down into a cave system and mushrooms spread everywhere. I recall one on his workbench and furnace. These are deemed solid blocks, so I think mushroom spreading applies to them as well.