I noticed the following addition to mono-env.eclass would be needed to let us kill go-mono.eclass (setting now obsolete SRC_URI and build phases relying on base.eclass)
Patch attached
--- mono-env.eclass~ 2013-05-24 20:44:55.000000000 +0200 +++ mono-env.eclass 2013-05-24 21:18:01.000000000 +0200 @@ -9,6 +9,8 @@ # @DESCRIPTION: # Set environment variables commonly used by dotnet packages. +SRC_URI="http://download.mono-project.com/sources/${PN}/${P}.tar.bz2" + EXPORT_FUNCTIONS pkg_setup if [[ ! ${_MONO_ENV} ]]; then