Market research firm NPD reports that Apple's iPod shuffle accounts for 58.2 percent of the United States flash-based MP3 player market in March, up from 43 percent in February.
NPD also believes that it won't be easy for competitors to overtake Apple's share. It's not just a matter of offering extra features and competitive prices.
You don’t walk into an existing market and grab nearly 60% in two or three month (at least not usually), and a big part of how this happened is that Apple’s self-contained system (i.e. iTunes and the iTunes Music Store) which “encourages” users to stay within it (you can bet that a lot of iPod shuffle buyers already owned a regular iPod or iPod mini), combined with an incredibly uncommon amount of cultural cachet that is impossible to buy.