There was an interesting thread last week on a list I subscribe to about how to best launch a major web site redesign. I ended up writing a white paper in response on how to best prepare for a web site launch from a technical standpoint: Five Critical Steps for a Successful Web Site Launch.
There was also a discussion of whether you should market the new site in advance or launch it quietly without fanfare. In my view, the decision should be driven by your overall goals and confidence in your timelines.
If the new site is the embodiment of a major initiative, making a splash with the launch may be in order. However, if you are not highly confident in your ability to stay on schedule, the soft launch will ameliorate a lot of potential risk.
Finally, when you do market the new site, focus on the new value it provides to visitors rather than the fact it is new. This sounds obvious but it is easy to lose sight of after the organization has invested so many resources in the effort.