I've been testing a setup where a JeeNode connected to an Ethercard not only receives data from remote radio nodes, but also sends commands to them. My sketch is based on the Ethernode server.
It seems that immediately after serving some web page data, the JeeNode enters a state where rf12_canSend will return true, but calling rf12_sendStart will not actually transmit any radio data.
Putting in more calls to rf12_recvDone didn't help, but adding a short delay (100 msec) between serving the web page and invoking rf12_sendStart seemed to fix the problem.
JCW, let me know if you want me to send you the sketches to reproduce the issue; they're kind of long to post here.