i say there is no need for maildrops on the at except in maybe a few places (fontana dam for example). the reasons go on and on, expense, planning time (it is time consuming, but you can do it in a week if you are effin crazy like me), and most of all, you get so sick of the damn crap, no matter how hard you tried to keep it interesting. there are so many markets on the at. big ones. there is not much you will not be able to buy. dont send yourself dozens of boxes of mac and cheese or ramen. there will be a store right next to the PO.
having said that, may i suggest something that many people cringe at, training. take the time you would be repackaging powdered milk (messy) and test your gear, and get your legs ready. i completely agree with ross, if you just show up at springer and walk north, it will come together naturally, even more so if you are not tied down by an itinerary or a support person back in grand rapids who has a fulltime job and lost that third package hes so sorry.
and it will come together so much more smoothly if you are just in a little bit of shape, your shoes are comfortable, and your rain system is flawless. so return the pallet of peanuts and walk as much as you can, gradually increacing the length, difficulty, and the wieght you walk with. my goal is to be able to do 10 miles in four hours with a full pack over real terrain. those days off in between walking days are what prevents injury. and being physicaly prepared eliminates a whole myriad of problems starting hikers experience, problems that force some to quit. its my suggestion. i have made every mistake in the book in this respect, so i speak from experience.
but one thing you will undoubtedly learn on the at is how to wing it and deal with situations you could never have predicted. have faith in that.
milo