The Parisian public libraries are making more steps towards computerization. It is now possible to search for books and check the status of a user account. See here.
It is apparently not possible to search in a selected number of libraries; also, there is no way to navigate through a Yahoo-style tree of topics (even though the topics are definitely hierarchical), but still, it is a good start.
There were also rumors that it would be possible to put an option on a book currently checked out but I couldn’t find a way to do that.