On Ubuntu 10.10 when I run the command
sphinx-build -b epub -d _build/doctrees . _build/epub, I see the following output:
Running Sphinx v0.6.6
Sphinx error: Builder name epub not registered make: * [epub] Error 1
It could just be a missing package, but I don't know which one. The sphinx documentation page doesn't say that the epub builder is only available in a later release.