I looked through the code you attached and you're totally correct, the return response from pachube is not the problem. I didn't go into depth looking at your state machine, but the cursory look I did didn't turn up anything that would cause me suspicions. I haven't worked on the RF stuff you're using so I don't know about that. Net, nothing.
Since you're catching it after a timeout, is there something on the RF that can tell you anything? A receive light or something? The idea about printing where you last were is not a bad idea, it may help to see what the last thing that happened is. Although the state machine could complicate the output too much. One other thing, you have a nice state machine for handling the internet connection, could something like this help you with the RFM12 interaction. It seems this is the device that is failing to respond and it could be an interrupt interaction where the ethernet board snatches the data away from the RF; this might require you to have a failure case for the RF parts.
Recovery is possible using the watchdog timer, but that solution sucks since you're hanging up so quickly.
Maybe someone familiar with the pieces I haven't worked with will speak up.