According to an article at SPOnG the upcoming Xbox 360 game console will have a variety of downloadable game demos available online the day it hits the stores. The content would be available via the Xbox live service. Microsoft is expected to introduce the service in the upcoming pre-E3 event.
The current "pay once, play forever" scheme will be around for quite a while, but game developers are looking more and more into concepts that "lure" people into paying, for example, a monthly fee for playing a game. Online multi-player games are just the beginning. Purchasing extra content to games will be the next step. The Sims games series is a prime example of this. However rather than buying a new disc for every added part, people will be able to purchase the content online and "plug" the extra content into their pre-existing game. Eventually you might be able to download, for example, a first level of a platform game for free, and then purchase additional levels as you go on.
Source:
SPOnG












