As rumoured, Onebox is a music search feature that appears inside Google search that will give you not a little 30-second clip, but a full song play.
Basically, as you can see in the picture, you will see play buttons alongside different songs when you get search results. If you press play, a popup from MySpace (iLike) or Lala appears, letting you play the full song and giving you buttons to buy it or get more info. You will also have the chance to check out the band on other services/sites, including Pandora, iMeem and Rhapsody. (As you might imagine, there's no mention of iTunes in all of this.)
In essence, Google isn't playing the music at all. It's up to MySpace and Lala to manage the rights of the full-song playback business and to serve up the content. For more info, check out the YouTube video or Google blog. They say they'll be rolling it out to US Google users over the next day. [Google]