I am looking for an alternative to wired networks for MRMP* and this looks promising!
if ( millis()-last_serial > 10 )
client.sendnow();
I suspect that will cause a very long window if millis() rolls to zero within the 10 millisecond window.
Granted it s a very unlikely as millis() only rolls over every 49 days!
*MRMP http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1232140631