Can I upgrade to Lion straight from 10.5.8?
At the moment it is not possible to upgrade to Lion without first installing the App Store, and Apple hasn't announced any plan to make that available on versions older than Snow Leopard.

From Apple's press release announcing Lion's availability:


  Users who do not have broadband access at home, work or school can download Lion at Apple retail stores and later this August, Lion will be made available on a USB thumb drive through the Apple Store® (www.apple.com) for $69 (US).


They haven't confirmed that this will work for upgrading for older versions, but given the price many people are assuming that this is the case.