[CentOS] Shadow passwords NOT md5'ed ?

Hello,
I have a very weird problem on a CentOS 5.2 system. A few months ago, encrypted passwords in the shadow file were MD5'ed - allowing for long passwords, rather than only 8 characters max.
Today, passwords are 3DES'ed only. No way to find how the system reverted to DES-encrypting. But what's more bothersome is we cannot get the system to go the MD5 way again, despite all our best efforts.
Authconfig says this :

pam_unix is always enabled
 shadow passwords are enabled
 password hashing algorithm is md5

So... where to look ?
Tia,
-- FdL