An important record business sales metric is the rate at which digital sales are overtaking physical. (Unfortunately, one not as fast as we'd like.) While not illustrative of the music market as a whole, it does provide some insight on consumer habits.
Here's a link to an interesting analysis by Mark Mulligan at paidContent of UK and US digital versus physical sales. In it, he offers up his explanation of the "Post Holiday Surge" as well as some background on the rate of change of the proportion of digital to physical sales.