I used the PAM module named poldi, which is part of GPG. On Ubuntu based systems, you can install the package libpam-poldi.

You then need to replace the 'auth' lines in any files in /etc/pam.d/ for which you'd like to authenticate against the smartcard (i.e. /etc/pam.d/kdm for the KDM login manager) with 'auth required pam_poldi.so quiet' (without the quotes).

Then, if it doesn't already exist, create the file /etc/poldi/poldi.conf, and add the line:

wait-timeout 3

To make poldi time-out after 3 seconds if no card is present.
You can test that this is working using:

$ poldi-ctrl --test

...with no card in the reader, and it should tell you so after the timeout.

Of course, these are just scribbles. The Poldi documentation is good.