samba - May 2008 - winbind, mod_auth_pam, and plaintext passwords

We have a working samba file server using winbind to authenticate with a
Win2003 server in native mode.
[2008/05/10 18:22:54, 5]
nsswitch/winbindd_cm.c:set_dc_type_and_flags(1651)
  set_dc_type_and_flags: domain STARTREK is in native mode.
[2008/05/10 18:22:54, 5]
nsswitch/winbindd_cm.c:set_dc_type_and_flags(1654)
  set_dc_type_and_flags: domain STARTREK is running active directory.

I now want to allow the apache web server (running on the same machine
as samba) to utilize winbind to authenticate users with domain
credentials. I have installed and configured apache with mod_auth_pam.
When I access a protected website I get a login box but it doesn't allow
me to login with my domain user/pass.

The apache log gives the following error:
[Sat May 10 22:47:20 2008] [error] [client 192.168.1.48] PAM: user
'matt.humrick' - not authenticated: User not known to the underlying
authentication module

This along with an strace of apache shows that winbind is being used via
mod_auth_pam for authentication with no obvious errors. Tcpdump also
shows packets being exchanged between winbind and the AD Windows server.

The following error appears in the winbind log:
[2008/05/10 22:39:09, 6] nsswitch/winbindd.c:new_connection(628)
  accepted socket 19
[2008/05/10 22:39:09, 10] nsswitch/winbindd.c:process_request(314)
  process_request: request fn INTERFACE_VERSION
[2008/05/10 22:39:09, 3]
nsswitch/winbindd_misc.c:winbindd_interface_version(491)
  [31171]: request interface version
[2008/05/10 22:39:09, 10] nsswitch/winbindd.c:process_request(314)
  process_request: request fn WINBINDD_PRIV_PIPE_DIR
[2008/05/10 22:39:09, 3]
nsswitch/winbindd_misc.c:winbindd_priv_pipe_dir(524)
  [31171]: request location of privileged pipe
[2008/05/10 22:39:09, 10] nsswitch/winbindd.c:process_request(314)
  process_request: request fn PAM_AUTH
[2008/05/10 22:39:09, 3] nsswitch/winbindd_pam.c:winbindd_pam_auth(751)
  [31171]: pam auth matt.humrick
[2008/05/10 22:39:09, 5] nsswitch/winbindd_pam.c:winbindd_pam_auth(764)
  Plain text authentication for matt.humrick returned
NT_STATUS_NO_SUCH_USER (PAM: 10)

I get a similar plaintext authentication error with wbinfo -a:
wbinfo -a matt.humrick%xxxxx
plaintext password authentication failed
error code was NT_STATUS_ACCESS_DENIED (0xc0000022)
error messsage was: Access denied
Could not authenticate user matt.humrick%xxxxx with plaintext password
challenge/response password authentication succeeded

So, challenge/response authentication succeeded but plaintext
authentication fails. This appears to be a configuration issue to me.
Obviously apache gives a plaintext user/pass to winbind vs. the
challenge/response method used by an WinXP client (which is working
fine). What do I need to do to allow apache to authenticate with
winbind?

I've read through the smb.conf man page and looked at several settings
relating to plaintext passwords. However, I'm a bit confused as to when
these settings should be used and whether they will break the existing
functionality between the WinXP clients, winbind, and Win2003 AD server.

Thanks,
Matt