Using --without-libiconv gives the message about --with-libiconv must have a directory argument.  The attached patch (for configure, not configure.in) fixes this and requires --with-libiconv to have an argument (possibly not what was intended).

diff -rc samba-3.0.23c/source/configure hold/source/configure
*** samba-3.0.23c/source/configure      Thu Aug 31 12:57:32 2006
--- hold/source/configure       Tue Sep 26 14:15:27 2006
***************
*** 29818,29841 ****
  if test "${with_libiconv+set}" = set; then
    withval="$with_libiconv"
  
!   if test "$withval" = "no" ; then
      { { echo "$as_me:$LINENO: error: argument to --with-libiconv must be a dir
ectory" >&5
  echo "$as_me: error: argument to --with-libiconv must be a directory" >&2;}
     { (exit 1); exit 1; }; }
    else
!      if test "$withval" != "yes" ; then
        ICONV_PATH_SPEC=yes
          LOOK_DIRS="$withval"
       fi
    fi
  
  fi;
  
  for i in $LOOK_DIRS ; do
      save_LIBS=$LIBS
      save_LDFLAGS=$LDFLAGS
      save_CPPFLAGS=$CPPFLAGS
-     ICONV_FOUND="no"
      unset libext
      CPPFLAGS="$CPPFLAGS -I$i/include"
  
--- 29818,29844 ----
  if test "${with_libiconv+set}" = set; then
    withval="$with_libiconv"
  
!   if test "$withval" = "yes" ; then
      { { echo "$as_me:$LINENO: error: argument to --with-libiconv must be a dir
ectory" >&5
  echo "$as_me: error: argument to --with-libiconv must be a directory" >&2;}
     { (exit 1); exit 1; }; }
    else
!      if test "$withval" != "no" ; then
        ICONV_PATH_SPEC=yes
          LOOK_DIRS="$withval"
+      else
+         LOOK_DIRS=""
       fi
    fi
  
  fi;
  
+ ICONV_FOUND="no"
  for i in $LOOK_DIRS ; do
      save_LIBS=$LIBS
      save_LDFLAGS=$LDFLAGS
      save_CPPFLAGS=$CPPFLAGS
      unset libext
      CPPFLAGS="$CPPFLAGS -I$i/include"