Joe Sullivan sent in vorbis.js (GitHub: itsjoesullivan/libvorbis.js, License: MIT), a project based on several others that produces Ogg Vorbis from PCM data. It uses Emscripten, and has an event-based API. There's a full example in the readme, which demonstrates how to configure the necessary memory buffers and encoding options.
Joe has also made a project called iago that can stream Ogg using web workers.
I've seen people on Twitter running the Linux version, but people are still dubious about Atom's performance and the 2 MB file limit. Making it open source seems like a solid first step to creating sustained interest, but it has a Google Wave feel about it.