Debian linux running a 2.6 kernel

To disable IPv6 you need both the following lines in your /etc/modprobe.conf file

alias ipv6 off
alias net-pf-10 off

Thanks to the guys on the #debian freenode channel for help with this.

Update

Moved the config to /etc/modprobe.d/local

Update

This stopped working on 2.6.18 on debian etch. Now my /etc/modprobe.d/local now contains just

blacklist ipv6

Apparently - in some cases this may still not work - in that case it is possible to try

install ipv6 /bin/true