Indeed I have been to one. Coffee Creek Center in Northern Indiana.
http://www.coffeecreekcenter.com/ I was there while the project was still in its late developmental phases but on a whole it was a wonderfully nice community.
It was shoe horned into an area that still showed signs of standard development and included a constructed wetlands and many natural features. I can't say that all the houses suited me or that I truly thought it was walkable in a year round sense, but it was more quaint, quiet and serene that most suburban tract home neighborhoods today.
There were attempts to bring amenities close to the residential areas, but there wasn't enough to support an entire community.
Its a great concept and when taken seriously it can have an impact, but as with any idealistic utopian type solution, there are short coming to the execution of the plans.