According to this post on the 3d Realms website, the previously mentioned content that was in development for Prey will indeed be free for PC users and Xbox 360 users as well. It was rumored and discussed in the past that the content would have more than likely been free on the PC, but not free on the 360. Well, those rumors were junk. Free content for all!
Included in the free content update will be brand new multiplayer maps and several new skins for players to choose from. There may or may not be additional content involved here, but 3D Realms isn't saying much. In fact, they aren't giving a release date either, just the normal "when it's done" spiel they seem to give pretty much everything they work on. I mean it isn't like they are working on any other high profile game that has a "when it's done" mentality!
In any case, as soon as a firm release date is set, or more information on what exactly will be included in this content update is released, you can be sure I'll post about it. In the meantime, don't forget to turn the gravity off.
Link: 3D Realms