However, while you can't yet drop an XNA game window onto a .Net form, you can create one separately. So that's what I'm doing.
That's my dead simple animation editor. You draw boxes around each frame you want, and then set the frame's origin. (In this case, I mean pivot point for rotation.) This information will be saved out into an xml layout that I've already built into the content pipeline. I'll be building a simple sprite editor next, then follow that up with a level editor.