MS Enterprise Library Open Source
Enterprise Library 4.0 has a variety of new and updated features. This more permissive license provides the long-sought ability to customize the internal workings. Also, according to Miguel de Icaza, Enterprise Library 4.0 is now able to be used with Mono. The release of source code will facilitate greater reuse of these building blocks on Unix with Mono.
Miguel did not say that EntLib is compatible with Mono, only that it's license is.
He answered to a comment that Unity works on Mono, but that's just a part of EntLib (and has always been Ms-PL, afaik)