Microsoft has decided to make some of the XNA games available for download to non-developers. This is a FREE preview to ALL Xbox LIVE members. The necessary XNA game player is available from the Xbox Live download menu and the XNA games itself can be downloaded from within the XNA Game tap that will be created in the Games section of the Xbox interface.
This feature has been available for people that had installed the XNA Developers tools on their PCs allowing them to stream their own applications to the Xbox360. By making available the XNA options to non-developers Microsoft has perhaps made a very smart move, give the XNA good PR.
Of course the supplied XNA games are time limited and remind me very much of the other game demos that you'll find on live. But with a few differences:
1) they are time limited
2) they can be made by regular folk - or could be as some of the XNA demos actually seem very early versions of commercial games

Now don't think that by installing XNA dev software - which is pretty much free (!!!) by the way - you'll be able to have your own programming efforts on XBox Live any time soon. For that to happen you'll need to join the XNA Live service costing around 99 USD a year if I am not mistaken. It's also possible to have 4 month-memberships costing less. And the ability to develop software for Windows, Zune (of course not available in Europe) and the Xbox360 will be yours. I don't know if this will make more XNA games available from the Live-interface or leave the non Microsoft sanctioned accessible content to developers only. I am betting on the latter as Microsoft doesn't want the regular user to crash their xboxes.

Microsoft does really show that the Xbox360 might not be that hard to develop for using the provided tools. At the same time it does make the 360 accessible to the hardcore programmer wanting to program for the 360 in a very legal way - as they won't have to chip their machines in order to gain the ability to run 'unsigned code'.

Here's a small video of me trying out some of the public XNA demos.