Like most things pertaining to bbq, you'll find lots of different opinions. The only thing I'd use green fruitwood on would be pork shoulder. I've used green peach wood for pork butts, and the bbq got gobbled up every time. However, I think I do prefer the flavor better after a little seasoning, like 2 to 3 months or so.
Seasoning is much more important IMHO for nutwoods which are much more dense; most folks saying six months but some experts say that three months is long enough. If you let the softer woods season too long though, they just won't smoke very long and they'll burn up very easily if you have the dome off for long at all. Basically, wood has to have some moisture in it to be good for smoking with, but it's certainly not at it's best green.