Just the rules, and a bit of math combining one-ways into RTs. Here are the full T&Cs from the offer email (key line in bold):

Terms and Conditions: Offer valid for select Alaska Airlines Mileage Plan members only. Triple Mileage Plan Miles valid only for paid tickets on non-stop flights operated by Alaska Airlines between Seattle and Boston between January 5, 2006 and April 4, 2006. Triple Miles offer limited to four one-way segments between Seattle and Boston per Mileage Plan account. To receive Triple Mileage Plan Miles, registration is required and must be completed prior to your first qualifying flight. Tickets must be purchased by January 31, 2006. Miles for qualifying flights must be posted to an Alaska Airlines Mileage Plan account. Bonus Miles not valid toward MVP or MVP Gold qualification. All terms and conditions of the Mileage Plan program apply. Offer subject to change without notice.