The 2.6 Linux kernel does not respond to multicast ICMP Echo requests by default. This is a setting in /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
Test it:
# cat /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
1
#
Set it to 0 for ping replies:
# echo "0" > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
# cat /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
0
#
