Well, we have used this model and it works pretty fine.  There were glitches initially but the framerok is being stabilized as the new versions are out. 

Mono is an open source implementation of Microsoft’s .Net Framework based on the ECMA standards for C# and the Common Language Runtime.

Cross Platform? : This framework is not just for Linux.  Mono runs on Linux, Microsoft Windows, Mac OS X, BSD, and Sun Solaris, Nintendo Wii, Sony PlayStation 3, Apple iPhone. It also runs on x86, x86-64, IA64, PowerPC, SPARC (32), ARM, Alpha, s390, s390x (32 and 64 bits) and more. Developing your application with Mono allows you to run on nearly any computer in existance.

Managed Add-in Framework

The assemblies for these segments are not required to be in the same application domain. You can load an add-in into its own new application domain, into an existing application domain, or even into the host’s application domain. You can load multiple add-ins into the same application domain, which enables the add-ins to share resources and security contexts.
The add-in model supports, and recommends, an optional boundary between the host and the add-in, which is called the isolation boundary (also known as a remoting boundary). This boundary can be an application domain or process boundary.

The contract segment in the middle of the pipeline is loaded into both the host’s application domain and the add-in’s application domain. The contract defines the virtual methods that the host and the add-in use to exchange types with each other.

To pass through the isolation boundary, types must be either contracts or serializable types. Types that are not contracts or serializable types must be converted to contracts by the adapter segments in the pipeline.

The view segments of the pipeline are abstract base classes or interfaces that provide the host and the add-in with a view of the methods that they share, as defined by the contract. Source : MSDN

Looking for the guidelines and best practices for application designs using .NET framework and associated microsoft technologies? Here is the best resource to start with – the application architecture guide 2.0 – from patterns and practices.

This gives you detailed description of various application types, the possible design methodologies and the pros & cons for each design approach.  Well, I believe each application will have it’s own requirements that impact the design decisions.  Understanding this document can make your design decisions a lot easier.

Happy reading !