The seven-day delay is important since Timeline is your new profile: it has all the content of your old profile, plus a lot more, in a completely different layout. You can choose to publish your Timeline at any time during the review period. If you don't do so, your Timeline will go live automatically after seven days.
Last month, Facebook started by pushing out Timeline in New Zealand, with the goals of gathering further feedback and to measure the new feature's speed and other types of performance. The test apparently went well, as the social networking giant deemed it ready for the rest of the world to optionally switch to, both on desktop and mobile. Now the company is pushing it on to everyone, whether you want it or not, although either way you'll still have seven days before the change is permanent.