Spotify, the (mostly) free music streaming service is launching in USA today.

Because some readers may be concerned by the lack of a Windows Phone 7 client, I thought it would be helpful to remind people that a Spotify app for Windows Phone 7 has in fact already been shown off some months ago at MIX 11 (video via winrumors.com).

The app, for Mango handsets, will of course address the biggest issue with non-Zune apps – background streaming.

Spotify in US offers:

• Spotify Free – the unsurpassed free music service. With an invite, enjoy on-demand, buffer-free access to over 15 million songs on your computer and great social features. Manage your own music files through Spotify, and sync with your cellphone or iPod. Features occasional advertising.
• Spotify Unlimited – all the special features of our free service, but with uninterrupted, ad-free access to Spotify on your computer. All for only $4.99 a month.
• Spotify Premium – the top-of-the-range Spotify experience. Premium gives you access to all the music, all the time. Listen online or offline, on your computer, your cellphone and a whole heap of other devices. Enjoy enhanced sound quality and access to exclusive content, competitions and special offers. Premium costs just $9.99 a month. (That’s the equivalent of a couple of fancy coffees.)

The free service is currently invite-only, but the anyone can sign up for the pay service now.

Read more at Spotify here.