It's pretty easy until you pass the halfway mark. 
Cakes that get smaller at the bottom are a bit tricky, but really, the technique is the same as for a regular round cake. Whenever you get to the bottom, you're going to have some gathers and folds. With a ball cake or something that tapers down, you'll just have more of them.
Lay the fondant on the cake and start smoothing around and around, starting at the top and working slowly toward the bottom. Move down just a bit at a time and work all the way around the cake before moving down further. As you near the bottom and get folds, gently lay each fold open, then press the flat bit of fondant to the cake to adhere. Keep going around -- open a fold, press it to the cake, open a fold, press it to the cake. As long as you work down the cake evenly all around, it's not tough.
The problems come in when folks glue the fondant down from top to bottom down one swipe, then move horizontally to the next section. You just wind up pushing all those folds and drapes to the back, and when you get there you've got no hope of getting rid of them.
Thats a tough one,I covered a soccer ball w/ mmf but I did it in pieces (black and white) , it was pretty easy. Maybe you can find a way to divide the earth up and do it in pieces, or just do the whole thing in blue and use the "continents" to cover up any folds or creases that you may have.
Thanks for the replies. I will have to do a practice cake before this one is actually due in December just to make sure I can get it smooth. I am counting on the continents covering any creases I can't get out. I will definitely remember to go around the whole cake getting lower with each pass around.
BKeith i am so smitten 
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