See: Thank you, Lindens
Bottom-line: the server-side must serve more than HTTP.
All these proprietary servers, however, are a bottleneck for the massification of 3D content on the Web. Just like for the Web itself, we need open source 3D scene servers. At the very least, we need open, standard protocols, so that people can focus on developing content without having to reinvent the network connectors, and the servers, from scratch all the time. (But with open standards come open implementations, so the result is the same: we need open source servers). Yes, we need an Apache for RT 3D scenes. Hmm, where did I hear that before…?