One of the most interesting new feature in Silverlight 4 is the COM interoperability functionality. This enables developers to access other components on the users machine (if the user allows the application to have elevated permissions).

There is a few examples out from Microsoft on how developers can access Microsoft Excel through it’s COM interfaces and pass data between the Silverlight application and Excel (and the other way around). This is quite cool but I wanted to develop my own COM classes and hook them up with a Silverlight mapping application.

So what I have done here is to build a mapping application with ESRI ArcGIS Server where the user can access data from ArcSDE (and also update the data). This can obviously be done by webservices (or the WebADF) today but in this example this is all happening on the clientside which is quite cool! This is just an example but the interesting thing now is that it is now possible to access ArcObjects (and other COM objects) on a users machine  from a web (OOB) application!

It might also be possible to stream down a COM dll from the server in a trusted application, register it on the client machine and then access it from Silverlight. I haven't tested this yet but I heard people mention it at the PDC. That would make it really simple to deploy your own COM classes and then do more stuff on the client machines!

Anyway, here is a video on how to integrate ArcSDE with your Silverlight application: 

I have a lot of ideas on how this can be useful to existing ArcGIS users (both desktop and server). You could potentially build COM classes with the ArcObjects functionality you want to use on the client side and have that streamed down and registered (and license with an Engine license perhaps?) on the client machine.
There is also other libraries that would be useful to get access to from a web mapping application.

These Out-Of-Browser applications (with elevated permissions) can be centrally managed through Group Policies (in Active Directory) which will make it very easy for IT admins to manage these applications within a Enterprise.

Links
Tim Heuer - Silverlight 4 Beta – A guide to the new features
Tim Heuer - COM Object Access in Trusted Applications (video)
The ArcGIS API for Microsoft Silverlight™/WPF™