To set configure Kmail for PGP/MIME, please see:
http://kmail.kde.org/kmail-pgpmime-howto.html

Here is a breakdown, for the lazy, of what must be done to get
PGP/MIME working:

prt-get depinst pinentry-qt

And as your GnuPG-utilizing user:

mkdir ~/.kde/env
echo 'eval `gpg-agent --daemon`' > ~/.kde/env/gpg-agent.sh
chmod +x ~/.kde/env/gpg-agent.sh

mkdir ~/.kde/shutdown
cat <<EOF>> ~/.kde/shutdown/gpg-agent.sh
#!/bin/sh
# This was pulled from: http://gentoo-wiki.com/HOWTO_KMail_gpg-agent_kde
# the second field of the GPG_AGENT_INFO variable is the
# process ID of the gpg-agent active in the current session
# so we'll just kill that, rather than all of them :)
kill \`echo $GPG_AGENT_INFO | cut -d ':' -f 2\`
EOF
chmod +x ~/.kde/shutdown/gpg-agent.sh