I noticed this problem a while back. There is (was at the time) a bug against su because, somehow, the compile-time flag needed to enable this logging had been removed. I noticed this change when I upgraded from hamm to slink.
My solution was to recompile su. It is in the shellutils package; you can just recompile su and copy it over, instead of recompiling everything, or otherwise you can make a new deb package and install it. Unfortunately, su is too sensitive a file for me to distribute my recompiled version. Carl
