A patent dug up by Siliconera details a new system conceived by Sony for limiting the content found in game demos. "Degradable" game demos, as described in a patent filed by SCEA, would initially include either a full game or a large amount of content, but that content would gradually disappear over playtime, to unlock again once the full game is purchased. Examples in the patent document include a weapon that becomes less effective over time, and race tracks that become unavailable one at a time.
Essentially, it's the DIVX rental system, but downloadable and for games. You know, DIVX, the system in which you "bought" a cheap disc that would only work for a short time unless you paid a fee to "unlock" it. That lasted less than a year because it was too popular, right?