My organization has a site built on ASP.NET. It uses ASP.NET to manage the users (students, class coordinators, and finances). However there is a small 'content' component of the site: about the class, about the organization, and some other content that will need to change and grow over time. However that part of the site is currently managed by Kintera which is $$. We want to get everything out of Kintera and into a CMS. ExpressionEngine is one being considered.
The site uses some iFrame's to pull content from the ASP.NET site into the Kintera managed content. This will likely be needed in an ExpressionEngine managed site.
These iFrames use content that is dynamically created from the ASP.NET site. The user's session will then need to be shared between ExpressionEngine and ASP.NET.
I am not involved in the ASP.NET site so can't say too much more than this. I will see if the developer of that site can provide any additional information or respond to your comments/answers.