As S60 is an open platform, presumably rival music services (or games services for that matter) could start selling content through their own downloadable S60 clients if they wanted to. Nokia couldn't stop them without closing the platform or running into anti-trust problems.
Your phone might turn into a high street of different service icons, and there might even be unofficial third party "personal shopper" applications that perform automatic price comparisons through the rival clients and then pick the cheapest option for a particular product.
In a way, this wouldn't necessarily harm Nokia because it's the hardware where the real money is made anyway, so anything which drives the sales of advanced (and pricier) hardware is going to benefit them overall.
It won't be like that forever though, one day all phone hardware will be cheap and low profit, which might be why Nokia are trying to move into internet services now so they have a next exciting big new profitable thing in the future.
"and if the networks will allow the music traffic through"
This raises an even more important topic, more important than any phone or service announcement. If the networks block traffic from what they see as a rival content service, even though the user is willing to pay the relevant data charges, that is surely an example of being anti-competitive, especially if the phone itself is locked to their network and the user is on a contract.
If it happens, I hope Nokia (and anyone else similarly blocked, including users) take it to the European Commission, because there's just no excuse for it, and it's time this kind of thing was stamped on. It harms customers, it harms competition among content providers, it harms content creators, and the only people it benefits are the networks.
How would people react if their broadband ISP blocked them from accessing iTunes because the ISP offered a similar but rival service? This would be even worse because at least changing ISPs is relatively painless and doesn't involve unlocking your PC.