According to a study done by In-Stat the market for audio players is expected to reach over 104 million units by 2009, up from 27.8 million units in 2004. Although Apple has a 30.2% share of this market, other leading manufacturers such as Rio, iRiver and Creative. The the percentage of US respondents owning a digital audio player has increased from 16% in 2001 to 25% in 2005.
This data suggests to me that the market for digital audio is just moving from early adopters to growth. We can expect to see the number of users and uses to expand at a near exponential rate. Every day I read of new applications for digital audio. The real limiting factor is imagination.