I have found a press release that states that Mono now support compilation for native client. It\'s unclear whether this is only available from the separate github mono fork, or
This post from March 2012 indicates that Chrome 19 has Native Client support for Embedded Mono
Versions of Mono for Native Client are now available for download in the SDK. These are compiled with glibc and the libmono comes as a dynamic library. Only pepper_19 and above has Mono available.
Note that you must keep your pepper_X and naclmono_X bundles in sync (ie, don't update one without updating the other) or you will probably be unable to run the resulting built nexes. New versions of Mono that match a given SDK should be available within hours of a new SDK build being available.
Only embedded Mono is supported in Native Client currently (http://www.mono-project.com/Embedding_Mono).