One of the features the CLR team is adding in Orcas is that we're providing a new model to help enable your application to host Add-Ins.  I've got a special interest in this set of features, as I always try to make my hobby applications pluggable for some reason, and I tend to end up writing a ton of infrastructure that, once Orcas ships, I will no longer have to.  The Add-In team has done the work to enable discovery, versioning, and most importantly (from my perspective at least :-) ) securely sandboxing those Add-Ins.

Recently, they've done a pretty big brain-dump of how their feature is going to work.  Some of the resources available are:

MSDN Magazine Articles:

Blogs

Jason's blog is interesting because he's decided to go through the steps to upgrade Paint.NET to use the new AddIn model, and discuss his experiences.  If you're intereseted in making your apps extensible, it's worth checking out this feature to see if it's made your life any easier.