Yes, X3D can be deployed through Darkstar.
Immediate Mode Interactive (www.imilabs.com) is developing an X3D-based game engine called Cosmic. I refer you to the Cosmic Blog page: http://cosmic-game-engine.blogspot.com/
I demo'd IMI's Cosmic Birdie video game based on the Cosmic game engine at the X3D Tech Talk - Siggraph 2006. All the content in Cosmic Birdie was created using RawKee, the X3D exporter for Maya.
IMI demo'd Cosmic Birdie using the Darkstar server in the Sun booth this year's Game Developers Conference.
If you look through the Darkstar documentation you'll notice that Darkstar supports HTTP requests for communication, so theoretically, it's probably possible to use the XMLHttpRequest/Ajax3D techniques outlined by Tony Parisi (http://www.aja3d.org/whitepaper/) to do multi-user environments with Flux and Darkstar.
Of course, licensing is another issue. And they last I heard from the IMI guys, Darkstar is still in the development phase, though supposedly, Sony has some sort of arrangement with Sun to deploy Darkstar as a game server for the PSP. I have no idea about the current status. You may want to check out the following URL: http://www.projectdarkstar.com
The Cosmic Birdie demo is available through the darkstar website.
So free-to-develop server software and open source X3D clients using AJAX techniques can now be applied to serious games (think training and scenario generation). There is a nice startup opportunity here or several.
A key thing to remember as Eric Maranne pointed out on the X3D list: don't sell syntax and languages. Sell applications. But the development costs are shifting away from licensing costs to server hardware and personnel. Figure out how to live within your means there, and as the DealNews guys proved, you can start and sustain a company long enough to become profitable even in the face of bad times (which BTW, times are good right now).