One of the advantages to music services like Spotify, Amazon MP3, and Google Music is that they provide a way to wirelessly transfer music to a smartphone. If you use Android and want to wirelessly transfer music between your phone and a desktop computer you can use Winamp, which now can transfer music from iTunes on Macs as well as the Winamp app for Windows.
I find it a real hassle to connect my phone to a desktop computer to transfer music from the computer, but I understand why people who have a large music catalog in iTunes might not want to upload all that music to an Internet service. To wirelessly sync the music in an iTunes library you will need to install the beta of Winamp For Mac Sync, which is a music player app for the Mac that synchronizes with iTunes.
You also need to install the latest version of Winamp for Android, which is available now in the Android Market. You will find a free and paid version of Winamp in the market, and both support wireless sync with Macs and Windows. The paid, premium version of Winamp is new to Android, and it removes ads as well as provide additional features like a 10 band graphic equalizer and the ability to customize the home screen.