Could this be the next big addiction? (Check the movie)
 

http://www.project-entropia.com

check out the movie on the right top of the page.

a virtual world with its own economy.

Lane, there have been virtual worlds with their own currency and economy for years. Active Worlds was the first online that I can remember; a fully-fledged online community that eventually started trading currency.
Then of course there's Ultima Online, I believe DOAC and Everquest both have their own currency structures.

This does look kinda neat though, like Unreal Tournament with strange animals once you get outside.

http://www.threshold-rpg.com has ruined hundreds of lives...

Okay, where's my crow? I reacted too hastily. I finally found the movie you were talking about (after viewing the photos and hitting 'exit' it took me to the page with the movie) and it's quite a bit more than Active Worlds or the other online communities (even those with currency) have ever achieved.
The ability to have a real "virtual" job and a salary that you can then convert into your own money and use here in the offline world - now that's something.
I'm looking for a terms of service now, my mind is exploding with ideas.