This is pretty big. Phil Schiller, on stage at WWDC, just announced that Lion will only be available through the Mac App Store. That means that no physical copy will be available. You can buy it once, and it works on any Mac you own. You will need approximately 4 GB of free space for the download, though.
It will cost $29 dollars, and be out sometime in July. For the developers, a new preview will be out today.