So I'm pretty sure Notch said he was gonna add a way to make Cobblestone turn into Moss Stone... How do you think it's gonna work out? (I'm not sure if he's gonna add it in 1.8, but he might, so I posted it in this board)
Maybe using Bonemeal on a Cobblestone block? Using seeds on the block? Just placing a Moss Stone next to Cobblestone and wait for it to spread? Discuss.
Then you put another block in way of the moss to stop spreading.
I don't know about you, but I like to mix mossy cobble with normal cobble in my buildings. Its a very nice look, like there are vines growing up the walls. If the moss spreads everywhere, it would ruin the effect, as would building other blocks around it.