Worldwide online music revenue from end-user spending is on pace to total $6.3 billion in 2011, up from $5.9 billion in 2010, according to Gartner, Inc. Online music revenue is forecast to reach $6.8 billion in 2012, and grow to $7.7 billion in 2015. By comparison, consumer spending on physical music (CDs and LPs) is expected to slide from approximately $15 billion in 2010 to about $10 billion in 2015.

And Apple is leading the way.