Apple doing it and Spotify doing it are two completely different things, traffic-wise. Assuming that Apple is basing their streaming solution off of Lala's model (not a given, but why else acquire them?), we're talking about giving every iTunes user streaming access to their entire music library, including songs that weren't purchased through iTunes (which would require a serious amount of cloud storage). Spotify reached one million users sometime last year; there are how many millions of iPhones and iPod Touch's out in the world?