Currently My Phone (Beta) is live (only for windows mobile v6).

You can install an app on your Mobile Phone by going to http://myphone.microsoft.com/install.
You will then be offered to download the installation file.

After install you will get an application named “My Phone”.
You will need an live id to sign up. After filling in your credentials it start to sign in and configure your My Phone service.

After you synced, you can access your photo’s, video’s and music on your My Phone Page. You could also access your calendar, tasks and contacts if you don’t sync with an exchange server.

Keep in mind that it could be that you’re cell phone provider won’t be that happy, when you’re gonna sync 4 gig’s of music files