Code access secirity, or lack there of, has been one of the complaints against Mono.
Mono CAS configuration tool
Benjamin Wootton has been working on Mono's Code Access Security (CAS) support. CAS is a way of creating arbitrary "sandboxes" in the .NET environment. The idea here is to load and execute untrusted side-by-side with your trusted code code without compromising your application. Similar in spirit to Java's security model.
CAS is needed for the various "click-once" setups for downloading, running and updating applications over the web, but more advanced uses enable scenarios like running untrusted plugins in a trusted application (for instance, a file format importer downloaded from the network, or plugins for your application which you do not necessarily want to trust).