I think as we see "Digital Rights Management" evolve as it has with songs over the past 5 years, we'll see the same happen with eBooks. You download it once, have access to it forever (In theory, at least) and also have the option to own a physical-digital copy which can be used wherever on whatever you personally own.
I would caution anybody who buys DRM-encumbered applications, video, music or reading material to make sure that you know the terms and conditions of any "shrinkwrap" agreement. Also make sure you're either able to convert to other formats, or that you are comfortable being unable to do so. If you decide to buy music (for example) that can only be played on a proprietary media player, be sure that you're willing to do without those purchases if the media player is discontinued.