In a press release dated October 18, Microsoft and Dassault Systemes formally announced the launch of 3DVia:

REDMOND, Wash., and PARIS — Oct. 18, 2007 — Dassault Systèmes (DS), a world leader in 3-D and product life-cycle management (PLM) solutions, and Microsoft Corp. today launched Microsoft^® Virtual Earth™ -3DVIA (Technology Preview). This free online application, developed by Dassault Systèmes, allows consumers to imagine and create realistic 3-D models — such as of buildings and structures — then share them through Microsoft Virtual Earth and online communities.

The new, free, online application actually has been live since Tuesday the 16th, where it was also referred to in the press release announcing the final updates to the fall Live Search release, for Live Search Maps:

Innovative visualization and 3-D. Coverage of bird’s-eye and 3-D imagery has been increased to cover 80 percent of the U.S. population. In addition, nearly 200 complete cities are available in 3-D, and customers can now combine views of 3-D imagery with Live Search’s unique 45-degree bird’s-eye images, giving a complete and detailed geolocation experience. In addition, people who use Live Search Maps can create and share their own 3-D models of buildings through an alliance with Dassault Systemes.

Anyway, you can get to 3DVia (after first installing Virtual Earth 3D beta, by clicking on 3D on the Live Maps, and then Add a 3D Model from the Collections tab:

From there you install the app, and then you're on your own.  I've been playing a bit with 3DVia, but although the interface is pretty easy to get around, it takes both patience and practice, neither of which I have time for right now.  Ultimately the idea is to create 3D models by placing them in Virtual Earth 3D, saving them, and then sharing them as collections.  Pretty neat, with a lot of potential.