You must Register by August 31.
You’ll be upgraded to Silver Preferred within 14 days, though folks report it happens much more quickly. And the status lasts for 90 days once upgraded. US Airways normally charges $200 for this offer.
Naturally, this only works for people who don’t already have status (or how could the status be upgraded to Silver?). It also doesn’t work for anyone that has received a trial status since May 1, 2007. And under current rules you can only enroll in a trial status program once every 24 months.
If you fly 7,500 miles or 10 segments during the trial period then status will be extended through February, 2010. In the meantime, you’ll get priority check-in and boarding, waived baggage fees, and some chance of uprgrades.