The WSJ reports that Amazon is not only planning a digital music store, but also will be offering an Amazon branded portable music player.

Amazon is negotiating deals with the major music studios to license content for a digital music store to possibly launch already this summer.
Selling music CDs is a big part of Amazon's revenue. The shift towards digital delivery makes it necessary for Amazon to get into the game.
If a Amazon mp3 player is a good idea remains to be seen. Samsung is rumored to be the maker behind the Amazon mp3 player.
Remember the Napster MP3 Player? It was also made by Samsung. I do not think that the Napster MP3 player was a relevant aspect of Napster at any point in time.
Microsoft is rumored to also release a portable music player to battle the Apple iPod.