Nokia, the leading cellphone manufacturer and a longtime Microsoft rival in mobile phone software, said on Monday that it had agreed to use Microsoft's music formats on its handsets. In another advance for mobile music, meanwhile, Sony Ericsson Mobile Communications said that it would make Sony Walkman-brand cellphones that would work with customers' digital music collections on their personal computers and connect with music downloading services. At the moment, digital music is largely carried around on portable players designed strictly for music, like the iPod made by Apple Computer. But hardware, software, music and phone companies agree that there is a mass market - particularly among young people - for music on demand, sent over the air to cellphones. Hmmm, I'm not so sure about that ...