As most of you have discovered (or, more likely, were a part of), you may have noticed that the world has been regenerated and you can't get back to previously built stuff. Don't worry, we're working on it.
We've generated a fresh world using the 1.7 world generator and have left it in creative mode. We're still in the process of deciding exactly what we want to do insofar as the eventual game mode, but most of us are leaning toward leaving the default mode in creative, with a portal hub going back to the old world you know and love as well as a fresh survival instance created with the new generator. Both old and new survival instances will share inventory state with each other but not with creative mode. Since we've never really had a full-on creative mode instance post-beta that actually worked right (or that people visited), we may give this a try as the default game type along with some WorldEdit commands I'll be making available to everyone. That said, I may disable the more dangerous features that can get you into trouble, but for the most part, do whatever you want. Boatloads of TNT explosions are OK, but don't be surprised if I find what you destroyed and regenerate it. This is particularly true if it impacts server performance.
We have yet to actually re-implement the multi-world connectivity that existed previously, but you'll be able to freely go between creative and survival once we do. For now, just have some fun.