Well I have been subscribed for a while, but the list is rather quite :-)

Indeed... We mostly use the forum, at the moment...  I'd like to use the ML more, though...

Does this mean that the IFC interpreter is completely separate from the
STEP interpreter in Freecad?

Yes, totally. For me the interest of the IFC format is much more in the non-geometrical
data than in the geometry itself, so I was more interested in having all the IFC data at
disposal than an automatic way to build the geometry. But there is now an OpenCasCade
project called ifcopenshell ( http://ifcopenshell.org/ ) that does the opposite. I didn't look
much at it, but I believe it could easily be used in freecad, if someone was interested in
a better geometrical support.
Is there a table/list showing what IFC elements are supported yet and
what is missing?

Not really... IFC is wide, and arch objects in FreeCAD is narrow :) If you
look at my todo list ( http://sourceforge.net/apps/mantisbt/free-cad/view_all_bug_page.php?filter=1607 )
see the items marked as "Arch module" you'll see all the stuff I'm working on
and it will give you an idea. Basically I would like to have basic stuff like walls,
structural elements, slabs, windows, doors and roofs working until next release.

The question may be how to handle the fact that Freecad supports (some)
architectural elements, but also can be used as a pure geometrical
modeler. So there would probably objects without any building-related
information in a file as well as walls, stairs, windows...

This is actually a very interesting idea, I hadn't thought about that before. We could
"catch" the supported objects and let the others import as plain geometry... I'll think
how to do that
I have to admit that I did not even know that openplm existed. What I
liked about bimserver was that it somehow works with any CAAD (as long
as there is an IFC interface for it) and that it "understood" the data,
e.g. allowing search queries and even a graphical browser.

Yes, plus the fact that the guys behind it are really motivated and
working hard... 

I will try to have a look at how you are implementing architectural
objects. I have no experience with python at all, but maybe it is still
possible to get an idea at least ;-)

It is a good moment to look at it, it is still pretty simple and not too messy...
I hadn't coded anything either when I first looked at freecad, you might
find python coding in freecad quite easy, and even pretty addictive ;)