Yea, I don't know why they didn't keep their existing structure until the new one was built unless it was a money or lease thing.
Also don't know why you weren't compelled to enter. Freedom Park (in front of the former location honoring journalists who gave their lives) is really nice. The place was so high-tech and amazing already, I cannot even begin to imagine what they're going to come up with! If you're in DC, Freedom Forum continues to hold great events.
Updates on new museum: http://www.newseum.org/newseum/newseum2006/index.htm