ASCAP (American Society of Composers Authors and Publishers), believes that every time your musical ring tone on your cell phone goes off in public, you're violating copyright law by "publicly performing" it without a license. They want their cut from the mobile phone companies. The mobile carriers argue that the owners of the musical compositions are paid for each ring tone download, but ASCAP claims that it's owed another royalty for the public performances (such as when your phone rings in a restaurant). Even if this were viewed as a public performance, that would also put you in jeopardy for playing your car radio with the window down, or having the radio on in a store.
Luckily, the chances of ASCAP winning this battle (based on the law) is slim to nil. EFF has more on this story.