What exact operating system are the computahs using?
In you ignore the wireless part of the puzzle, and plug the computahs directly into the switch on the back of the router for a wired LAN connection, do you have the same problem?
- Windoze XP firewall is blocking everything.
- Some other personal firewall is blocking pings (ZoneAlarm, Kerio, Norton, McAffee, Black Ice Defender, etc)
I've seen some really weird things with ping, where Windoze ICMP ping doesn't work, but Linux UDP ping does work. It's usually buggy firmware of some setting that has an undesired affect ( such as block pings from WAN).