...from: http://www.apple.com/macosx/uptodate/

If you've purchased a qualifying computer that does not include OS X Lion, you can upgrade to Lion free of charge. And if you recently purchased a qualifying Mac Server, you can upgrade to Lion Server at no additional cost. Check out the conditions below to see if you qualify.

OS X Lion Up to Date Program- How it works

Qualifying purchases between June 06, 2011 and July 20, 2011

Customers who purchase a qualifying new Mac Computeror an Apple Certified Refurbished computer from the Apple Online Store between June 6, 2011 and July 20, 2011 that does not include OS X Lion can upgrade to OS X Lion for free.

If you purchased your computer from the Apple Online Store, an Apple Retail Store, or an Apple Authorized Reseller, follow one of these links to participate in the program. Remember your order must be entered by August 19, 2011. Subject to terms and conditions.

Redeem from the Apple Online Store

If you cannot submit an order through the Apple Online Store, follow this link to participate in the program.

Qualifying purchases on or after July 21, 2011

Customers who purchase a qualifying new Mac Computeror an Apple Certified Refurbished computer from the Apple Online Store on or after July 21, 2011 that does not include OS X Lion can upgrade to OS X Lion for free.

If you purchased your computer from the Apple Online Store, an Apple Retail Store, or an Apple Authorized Reseller, follow one of these links to participate in the program. Remember your completed order must be entered within 30 days of the date of your purchase of a qualifying computer (described in this offer). Subject to terms and conditions.

Redeem from the Apple Online Store

If you cannot submit an order through the Apple Online Store, follow this link to participate in the program.