We’re targeting the first quarter of 2008, but the exact date really depends on feedback we receive from testers and the work we put into making sure we understand the feedback we receive. We’re making a beta available to more than 10,000 people in the next few weeks – that’s a critical step for us on the road to release, and we’re looking forward to the feedback we’ll get.
The SP1 beta testing group is currently quite small. MS will will gradually increase the number of testers until it is eventually opened up to the MSDN network. That will be at the pre-release stage.
This announcement isn't really news. Many have speculated that SP1 would indeed be released in the first quarter of 2008. However, MS has not made an offical announcement until now.