I noticed this when I was creating planters around the outside of some homes in my town on my SMP server. When you place upside-down stairs on grass blocks, the grass underneath will eventually decay into regular dirt after a little while like normal. However, I noticed that blocks do not decay if the upside-down stairs are made of oak, but do decay if the stairs are made of spruce. This is 100% reproducible. I tested this with other types of wood and proved that oak stairs upside-down do not decay grass into dirt, but all other types do. Here are my screenshots as proof:

Before: http://i116.photobucket.com/albums/o24/Arclese/2013-03-13_220718.png

After: http://i116.photobucket.com/albums/o24/Arclese/2013-03-13_220957.png

I only tested this in 1.4.7 on my server, but after searching for this issue on this site, I did not find anything and I assume this went under the radar into 1.5.

Thanks for reading, guys!