rsiegel, Thank you very much for your reply. I am in the process of chasing all of the related posts and references down. I suspect something useful will come of it.
Our hardware is a custom board based on the ATXmega128A3, which is quite a work horse. We are using a prepackaged Digi-International transceiver in the central office and XBee PRO 900H devices in the field. The field units are in weatherproof housings, battery operated, and solar panel charged. An Arduino solution would be somewhat redundant since we can already communicate with a PC. We are pretty far along in the development cycle and have a messaging system based upon cellular connectivity. I think our easiest way into the Cayenne system may be a custom gateway that can deal with our protocol on one end and Internet connectivity on the other.