This promotion actually started the beginning of May prior to my blog so I forgot to mention it, but there is still a lot of time left. If you reside in Northern California, specifically in the zip codes that begin with 923, 932, or 934-961, and are not a member of Alaska Airline's Mileage Plan, now is the time to sign up.
Enroll from this link and they will automatically start you off with 500 miles. And from now until October 1, 2011, new members who book and fly an Alaska Airlines roundtrip flight from Northern California to Hawaii will receive 25,000 bonus miles (That's already enough for another roundtrip award flight anywhere in the Continental US). New members who book an Alaska Airlines roundtrip flight from Northern California to anywhere else Alaska flies will receive 12,500 bonus miles. See terms and conditions in the link for details.
Alaska Airlines is trying to heavily promote it's Bay Area presence through this offer, and I would definitely try to take advantage of it. Bonus miles can only be earned once. Mileage Plan miles are actually quite versatile because even though Alaska does not belong to any major alliance, they have partnered with a number of global airlines, and you can redeem through them. Partners include Cathay Pacific, KLM, Delta, Air France, Korean Air, LAN, American Airlines, British Airways, and Qantas.