The US Portable Music Device Forecast, a new report from Jupiter Research, predicts that over 100 million portable MP3 players will be in use by 2011 in the US alone.

According to Jupiter, the iPod will keep its position as the most popular MP3 player.

“Despite the coming of Zune, the return of closed-loop digital music service-device combinations, and music phones on the horizon, the iPod should not lose significant market share in the next 12 to 18 months,” said research director Michael Gartenberg.

The study highlights flaws in MP3-ready mobile phone systems, noting that, though the devices will outnumber dedicated MP3 players by 2009, the actual usage of the phones for listening to music is “nascent in the US”.”US mobile phone carriers are underemphasizing or ignoring altogether the necessity of enabling users to sideload existing music collections onto a phone because they cant charge for it,” said Jupiter Research vice president and senior analyst David Card, who served as lead analyst for the report.

The report comes as Microsoft prepares to launch its highly-anticipated Zune player on November 14, just weeks after SanDisk and Apple rolled out major upgrades to their MP3 players.