As of this writing, it is a little painful to use pbuilder to create a Debian chroot on an Ubuntu host due to LP: #599695.

The easiest workaround I could figure out was the following:

$ cat ~/.pbuilderrc-debian
COMPONENTS="main contrib non-free"
DEBOOTSTRAPOPTS=("${DEBOOTSTRAPOPTS[@]}" "--keyring=/usr/share/keyrings/debian-archive-keyring.gpg")

And then you can issue:

sudo pbuilder create --basetgz /var/cache/pbuilder/sid.tgz --distribution sid --mirror http://mirrors.kernel.org/debian --configfile ~/.pbuilderrc-debian

The better way to fix this of course, would be to fix above bug. But this works for now.