Basically they promise not to sue over patents on implementation of the ECMA specs for C# and CLI. Mono will be splitting their source code distro into ECMA code and the rest (ASP.Net, WinForms, etc..) that isn't covered by the ECMA specs for those worried about potential patent claims on the later.

Miguel De Icaza's blog post: http://tirania.org/blog/archive/2009/Jul-06.html