After some discussions with the team they confirmed, that this will work with RTM, but there are a couple of limitations we should be aware of:

• We cannot really communicate between the hosted WPF page and the gadget.
• Therefore we don't have access to the JavaScript APIs provided by the Sidebar gadget API and
• we don't have access to the Sidebar events from within the WPF page hosted through an IFRAME in the gadget.
• And of course we have to be aware of Code Access Security, which shouldn't be an issue for enterprise-level applications (especially for those who are already using URL-launched .NET applications today:))

In the end I am very excited about the option of hosting WPF through IFRAME in a gadget because it's much easier creating a gadget that communicates with more complex web services in a much easier way than it is possible through the (in my opinion "limited") JavaScript APIs.

What does it mean? Well, everybody has the option! Create richer, WPF based pages for integrating with complex back-end applications and with other client applications like Outlook (as calling into Outlook or other client apps is much easier from a .NET based application than from JavaScript) or creating the nice, little, fancy JavaScript / DHTML based Sidebar gadgets...