[imp] using IMP to handle horde authentication

[Liam Hoekenga]

I'm currently playing around with using IMP to handle horde 
authentication for some of instances that aren't behind out SSO.

People are getting shunted into the Horde portal screen, tho, instead of 
seeing their inboxes.

the auth blocks from horde/config/conf.php look like..

$conf['auth']['params']['app'] = 'imp';
$conf['auth']['driver'] = 'application';

When users go to...

      https://mail.example.com/horde/imp/

they get the horde-esque login screen.  Then the location bar shows...

    https://mail.example.com/horde/imp/redirect.php

and then they end up at...

    https://mail.example.com/horde/login.php

I've tried setting hordeauth to both true and false in 
imp/config/servers.php in the appropriate server stanza.

What am I doing wrong?

Liam