Initially I tried to set up a 3D projection-matrix with a suitable camera position and switch between that and the 2D orthogonal projection as soon as I want my 2D quad to "tilt away". But that is never precise, and I need to work with weird camera offset positions that I am sure only work with the current screen resolution. So there must be a better approach. I started working with transformed vertices, and doing the transformation and projection myself, but don't achieve satisfying results with that yet either.
The Windows Desktop Manager (starting with Vista) does that all the time: all the windows are properly displayed without any "texture blurring", and it seamlessly transitions between 2D and 3D effects (a perfect exampled is Aero Flip). How do they do that?