Update: The Music section of the Market is now live! (Link). Access to Music in the Android Market (on-device) will be rolling out over the next few days on the server side (there is no new version of the Market to get the Music section). A new version of the Music app is out as well, check it out here.
Update 2: Here's the full video of the Google Music event held in Los Angeles today (if the embedded timecode doesn't work, skip to 29m30s for the start of the event):
Just minutes ago, Google announced the launch of the long-awaited Google Music storefront.