I couldn't find anything in the forums about this, but when I harvest the trunk of a tree the leaves don't degrade on their own. I've tried this out on a few different worlds since the Halloween patch and it comes out the same. I also don't mean that it happens in a short period either, I've went off for several 'days' and none of the trees had changed. I'm not sure if this was intentional or not but I liked the old system MUCH better IMHO.
Notch is trying to re implement the feature, but last time he tried (yesterday) it made an infinite loop cycle in some cases which leads to crashes and many problems. But he is trying to add the feature back in, so don't worry!
I'm still waiting for this to be put back in... I have a tree farm... and right now it is useless to me because the leaves don't degrade... There is just a giant massive floating blanked of leaves that prevent me from planting.