I'm also sitting with a project where this would be immensely useful. I hope someone pieces it together soon!

Hmm it seems that the wifi shield and UDP isn't up and running, even though the shield is advertised as capable of doing it.
http://arduino.cc/forum/index.php?topic=121503.0

Ok, that's also what I suspected. I will look into UDP packets and see if I can make it work. 

Well the title more or less describes my question.

I have two arduino boards, both with the wifi shields, which are currently communicating with cosm(pachube). This part works fine and the feeds are running as they should. What I am interested in doing is having each board react when the other board sends a new value to its feed. I know it's possible through a server using API's and POST/GET requests or something along those lines, but I was thinking if it's possible to simply send a ping to the other board's IP address and have it react to the ping? That would make my life a lot easier as I'm not that skilled at the server side of things.

Any help would be greatly appreciated! Thanks!