is there a server and client that supports it?
from http://www.matroska.org/technical/guides/faq/index.html
Will Matroska be streamable? Yes, but low bitrate streaming like streaming Vorbis, will always be better in Ogg. This is because their design is for different purposes.
They're referring to (at the time it was written) streaming Vorbis using Ogg, which is not at all an unreasonable thing to do. Ogg was originally designed to contain Vorbis, and thus Vorbis-in-Ogg is not particularly evil or difficult to deal with. Ogg only became seriously problematic when they tried to extend it to include things that it wasn't originally designed to be able to do.