Hi Pilou, well there seems to be several layers to the problem with Blender vertex normals.

The first layer is that the OBJ importer just ignores vertex normals from the OBJ file.

But even if that one particular thing is fixed, there are some more problems like whenever you enter edit mode, vertex normals are discarded and recalculated and even more problematic is that whenever you go to a render the vertex normals are again discarded and recalculated then too.

So just getting them imported is only one of the problems.

Here for example someone opened a bug that any vertex normals that were actually imported are discarded if you just go to edit mode and back out again without actually doing anything, and the bug was rejected:
http://projects.blender.org/tracker/?group_id=9&atid=498&func=detail&aid=26405

There was an extensive discussion of this on the forum here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4272.1

- Michael