Howtoforge posted a tutorial about restricting users to SFTP in Debian Squeeze

This tutorial describes how to give users chrooted SSH and/or chrooted SFTP access on Debian Squeeze. With this setup, you can give your users shell access without having to fear that they can see your whole system. Your users will be jailed in a specific directory which they will not be able to break out of. I will also show how to restrict users to SFTP so that they cannot use SSH (this part is independent from the chroot part of this tutorial).

  Restricting Users To SFTP Plus Setting Up Chrooted SSH/SFTP (Debian Squeeze)