Can't ping a machine - why?

Hopefully I've chosen the correct forum for this...

I have two MBPs, and two iphones in the house, connecting wirelessly through an AirportExtreme.

Out of all the devices, there is one MBP that can't be pinged even though it's online and works perfectly. It can see everything else, but nothing can ping it. Since noone can ping it (by IP), obviously it isn't DNS or other routing.

I've got the firewall turned off on all machines since I'm on a private network, so that isn't it either.

Can someone suggest what else I could try?

Thanks!