The Sims 3 team has released their Create A World tool, which was originally created for their in-house use in developing the various neighborhood/worlds.  I finally had some time to check it out over the weekend.  I'll be honest with you, I couldn't make heads or tails of it.  It's not an easy tool to start with - this is definitely something for power users!  I was expecting something more like the Edit Town feature or Create A Sim tool.   Instead, Create A World is more like the custom content editor that was released for The Sims 2 (which I also was thoroughly unable to master).

Create A World is a separate program, which you can download from the Sims 3 website here.  (Currently it is PC only, but I assume a Mac version is on the way?)  And fortunately for those who want to explore it more thoroughly than I did, there is both a walkthrough and a tutorial!

The walkthrough isn't a "walkthrough" in the traditional sense, more like a tutorial.  It's a collection of good advice, tips and tricks, and suggestions from La Shawn (Rain), the lead developer of the Create A World team.  The walkthrough gives you an overview of what you want to accomplish during each phase of worldbuilding, from choosing the basic terrain (mountainous, farming, etc) to advice on setting spawners (those points where collectible bugs, gems, and seeds appear in the game).

By contrast, what the team calls the Create A World tutorial is actually more of a user manual.  It is huge, and very detailed, and strangely enough has been formatted like the booklet that comes with your Sims DVD.  I suspect they plan to include a real-world version of the booklet along with the next expansion pack, and possibly to bundle it with the Sims 3 base game DVDs from this point forward.  At any rate, this 27 page booklet is dense with information, and incredibly helpful for the Sims fan who wants to dive into Create A World.

Meanwhile, while I was dinking around with Create A World and letting myself get confused and frustrated, other Sims players were getting down to brass tacks!  There are a bewildering array of worlds you can download from the Sims 3 exchange.  Someone has even recreated Strange Town!  

I have yet to download and install a custom world, but I gather they behave the same as what were originally called "Neighborhoods."  In other words, you would have Sunset Valley, Pleasantview, and Strange Town to choose from after that first loading screen.

In the end, Sims 3 Create A World is the same tool that the EA developers used to create the official Sims 3 neighborhoods, and the three worlds that were released with World Adventures.  It isn't necessarily easy to use (it was never meant to be), but it is very powerful and flexible.  And I have to commend EA for understanding that its users want to customize everything, and giving us the tools to do so.