When iTunes was first released it was available only on Macs. When they made the Windows version that is when it took off. iTune's history shows Apple is willing to develop for other platforms.
One thing Apple should do with iRadio is make Windows Phone and Android Apps. They still have a good partnership with Microsoft so there shouldn't be any issue making a Windows Phone one, and with Google's open store, nothing prevents them from making an Android one.
One thing Apple should resist the temptation of is to try to use iRadio as a lure to entice people to the iPhone. A new product would not do this, especially with Pandora and other services already so well established. However, if they make a good service with iRadio and make it available to as many as fiscally makes sense, this will only be a benefit to the consumer. Good competition always is.