It looks like there's a project in Seamonkey called DotNETEmbed for building .NET wrappers for the Gecko engine using Managed C++.
There's only some technical information provided in the Draft summary of the project. Does anyone know any more about this project? Embedding Gecko in .NET projects would be very cool.
The MONO project (which amais to bring .NET to not-Windows platforms) is doing something similar atm. Instead of providing an extra object (like Geckobrowser) they catch the calls for the Webbrowser object and use Mozilla to display them instead of IE. Nifty! :)