[Update: Microsoft starts to officially support Mono, so please simply use latest NuGet executable such as 3.5]
On openSUSE (12.2) it is easy to find that xbuild and Microsoft.Build.Framework.dll is available under mono-devel package,
1 lextm@linux-gtil:~> rpm -ql mono-devel | grep xbuild
Then it is obvious that Microsoft.Build.dll is not within this package,
1 lextm@linux-gtil:~> rpm -ql mono-devel | grep Microsoft.Build*
For Ubuntu, the package structure is different. xbuild is in mono-xbuild, while Microsoft.Build.* are split into several packages such as mono-xbuild, lib-mono-microsoft-build-framework4.0-cil, and so on. I have to use which xbuild and dpkg -S /usr/bin/xbuild to locate the package name, which is not intuitive.