Here's a very quick howto on disabling IPV6 on Debian (Lenny), this is not a new argument and neither an unknown solution, it's just how to disable IPV6 on Debian with less impact as possible on your machine config in "the debian way".
Sometimes you don't need IPV6: you've an already existing IPV4 net and you're happy with it, you don't want to waste memory or cpu cycles, you'd just like a very basic networking and setup.
There're a lot of way to do this, this mode is more "debian friendly".
Just place a file named for example "00local" in your /etc/modprobe.d directory, it should look like mine:
luke:/etc/modprobe.d# cat 00local alias net-pf-10 off
alias ipv6 off
These two aliases should disable IPV6, this is a quite common config used in other distro as well, easy to port if you upgrade frequently or change from a major common release to another one.
Hope it helps