Comment 4 for bug 1094278

for what it's worth: to make sure this wasn't a polkit thing, i made an action definition for this, and dropped it in /etc/polkit-1/actions:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policyconfig PUBLIC "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN""http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd">
<policyconfig>
<action id="org.freedesktop.policykit.exec">
    <defaults>
      <allow_active>yes</allow_active>
    </defaults>
    <annotate key="org.freedesktop.policykit.exec.path">/usr/bin/cu</annotate>
    <annotate key="org.freedesktop.policykit.exec.argv1">-l /dev/ttyS0 -s 115200</annotate>
    <annotate key="org.freedesktop.policykit.imply >true</annotate>
    <annotate key="org.freedesktop.policykit.owner>unix-user:ummelum</annotate>
  </action>
</policyconfig>

i ran it...
$ exec /usr/bin/pkexec --user ummelum /usr/bin/cu -l /dev/ttyS0 -s 115200
but alas...

according to /var/log/auth.log, the policy worked:
Dec 1 22:51:54 iugo polkitd(authority=local): Operator of unix-session:/org/freedesktop/ConsoleKit/Session1 successfully authenticated as unix-user:ummelum to gain ONE-SHOT authorization for action org.freedesktop.policykit.exec for unix-process:5798:3872562 [bash] (owned by unix-user:ummelum)
Dec 1 22:51:54 iugo pkexec: pam_unix(polkit-1:session): session opened for user root by ummelum(uid=1000)
Dec 1 22:51:54 iugo pkexec: pam_ck_connector(polkit-1:session): cannot determine display-device
Dec 1 22:51:54 iugo pkexec[6047]: ummelum: Executing command [USER=root] [TTY=/dev/pts/0] [CWD=/home/ummelum] [COMMAND=/usr/bin/cu -l /dev/ttyS0 -s 115200]

so this isn't a polkit thing.