If you can't wait to buy that Mac with OS X Lion pre-installed, go ahead and purchase it. According to Apple's website, you can get OS X Lion at no addition charge for Mac's purchased on or after June 6, 2011. You must request the up-to-date upgrade within 30 days of buying your Mac though.
The OS X Lion Up-to-Date upgrade will be 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.
When Lion becomes available in July, check the OS X Lion Up to Date program page for details on how to request your copy of Lion.