NetBSD 3.1, PAM and imap-uw

I've got quite a number of servers using NetBSD 3.1 with imap-uw. All 
standard stuff with nothing out of the ordinary (basic unix authentication 
only).

One server however will not authenticate users. The same users can use 
FTP, telnet, etc. with no problems.

/usr/lib/security, /etc/pam.d, /usr/pkg/libexec/imapd and OS version are 
all identical.

I've done ktraces of:

printf "a LOGIN user pass\na LOGOUT\n" | /usr/pkg/libexec/imapd

They do differ very slightly, but only (as far as I can see) down to the 
passwd files being larger on the non-working one (864 users vs 63 on a 
comparison server).

Compiling up imap-uw without PAM support gets it working, so it looks 
like a PAM problem, but I've run out of things to compare.

Any clues?

-- 
Stephen