If you purchase a Mac between June 6th and the launch date of OS X Lion (date not yet announced, but it’s coming some time in July) you will be able to download and upgrade to OS X Lion for free.
From a press release issued by Apple…
The Mac OS X Lion Up-To-Date upgrade is available at no additional charge via the Mac App Store to all customers who purchased a qualifying new Mac system from Apple or an Apple Authorized Reseller on or after June 6, 2011. Users must request their Up-To-Date upgrade within 30 days of purchase of their Mac computer. Customers who purchase a qualifying Mac between June 6, 2011 and the date when Lion is available in the Mac App Store will have 30 days from Lion’s official release date to make a request.