[sudo-users] sudo never prompts for password

[Donald Jenkins]

Hello all.
There is strange problem I can't deal with. I have two environments:
desktop with X11 and server. On desktop sudo prompts me for password
in usual way. On server - never.
Typical sudoers file I use on both machines:

# sudoers file.
#
# This file MUST be edited with the 'visudo' command as root.
# Failure to use 'visudo' may result in syntax or file permission errors
# that prevent sudo from running.
#
# See the sudoers man page for the details on how to write a sudoers file.
#

# Host alias specification

# User alias specification

# Cmnd alias specification

# Defaults specification

Defaults  timestamp_timeout=0
Defaults  requiretty
Defaults  secure_path =
/sbin:/usr/sbin:/bin:/usr/bin:/usr/local/bin:/opt/bin:/opt/sbin:/root/bin

# Runas alias specification

# User privilege specification
root    ALL=(ALL:ALL) ALL
user   ALL= /sbin/ifdown eth0,/sbin/ifup eth0

# Uncomment to allow people in group wheel to run all commands
# %wheel        ALL=(ALL) ALL

# Same thing without a password
%wheel  ALL=(ALL:ALL) NOPASSWD: ALL

People not in wheel group (here - "user") never getting password
prompt, resulting in NOPASSWD effect same as for wheel group, but when
I try sudo -n with their priveleges, sudo answers "sudo: sorry, a
password is required to run sudo". Same config on desktop working
perfectly, and I get password prompt.
sudo is built with PAM support, PAM settings is identical on both machines.
Both sudo versions are 1.7.2p7
Please point me at right way, I'm stuck with this.