Costing arbitrage between exchanges
nmat: i think you got the signs wrong in there somewhere. the fees will /decrease/ your effective selling price, and /increase/ your effective purchase price. so it should in fact be:

ExchangeA_sell * (1 - ExchangeA_fee) - ExchangeB_buy * (1 + ExchangeB_fee)


and also, you should take into account any deposit/withdrawal fees (E.g., when you withdraw via LR from mtgox, that's a 2% hit, when you use dwolla, that's 0.25USD fee, etc) that you'd incur when moving money between the exchanges.