Apple's iTunes Music Service sold an estimated 275,000 tracks at 99¢ each in its first 18 hours, Billboard reports.

The publication cites "major-label sources" for its claim. The music industry is waking up to its digital destiny, and the race is now on to develop a service for Windows - which Apple is already working on. Billboard reports "sources" who claim two labels have already reached deals with Apple for the Windows service.

Interviewed by Time, Apple CEO Steve Jobs downplayed the possibility of another firm filling the space. Jobs points to the company's investment in digital-distribution infrastructure, such as servers and technology, as well as the hardware and software required to make such a service happen.

He also mentions the prevalence of Macs in the music industry as part of Apple's success in creating the service.

"I don't think this is going to be so easy to copy," he says.