While Apple's Mac OS X Lion Up-to-date page
simply states, "Check back in July for details on the OS X Lion Up to Date program", their Press Release
on Lion has the following information....
"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 Lions official release date to make a request.
The webpage and press release seem a little conflicting, but it would appear that any Mac purchased from now on will include a free copy of Lion through the Mac App Store.