Wal-Mart is #2 for US music sales at 14% (that's a combination of both their online and CD sales) and Best Buy is third. Speaking of CDs, the aging format is still the overall top seller in the US and Wal-Mart is the top CD distributor. However, NPD expects that Apple's sales will equal that of CDs by 2010.
I know it's impossible to say what I'm about to without sounding like a grumpy old man, but here it comes anyway. For me, the tremendous thing isn't that Apple has commandeered the market so handily, it is the rate at which the distribution model has evolved. I'm only 38 years old, but as a kid I had a box of records. By the time I was in junior high school I was buying cassettes and in college I bought CDs.
Today, I can't remember the exact last time I bought music in a format I could physically hold in my hands. I'm glad the big wigs in the music industry are starting to get it. Now if only the TV execs would follow suit.