debian upgrade problem

I wanted to go from 'woody' to unstable (sid?) so I changed
my sources.list to point to unstable and then issued the command
'apt-get update' then 'apt-get upgrade'.

I have successfully downloaded all the new .debs but upgrade chokes
with the following message:

E: This installation run will require temporarily removing the essential
package libpam-modules due to a Conflicts/Pre-Depends loop.
This is often bad, but if you really want to do it, activate the APT::Force-LoopBreak option.
E: Internal Error, Could not early remove libpam-modules

Any tips?
-mike