Rob Faludi and I are working on a Processing library for Digi’s XBee Series 1 radios. Direct from Rob’s blog:

The library currently facilitates receiving single sample I/O packets in API mode, and returns an object that contains the analog values, digital values, sender’s 16-bit address and RSSI value. The next tasks will be to receive regular RX frames, issue AT commands and receive responses, issue TX frames and receive responses to those. We’d also like to support the XBee Series 2 radios, which have a similar API structure.

Here’s where you can download and learn about the library.