There are a few questions related to current FBO implementations that have come up repeatedly in this forum. At the SIGGRAPH 2005 OpenGL BOF, NVIDIA gave a short presentation that answers many of these questions with respect to “release 75” drivers. That presentation has been made available here:
I hope that it will help to answer some of your questions.
One thing to add is that support for EXT_packed_depth_stencil is already implemented and will be available relatively soon in public drivers.