Digi’s 802.15.4 firmware has “virtual wire” capabilities. That’s well described. With it, as said above, you use simple AT commands to define which digital pins are inputs on one XBee and these can be duplicated as outputs on some other Xbee. The pairing of the XBees is via the DH:DL address settings and the radios’ MAC addresses.
The criteria of when to send the current bits can be, as I recall, every x amount of time, after n samples taken at an interval you define, or when the bits change. I think you can do the same with the A/D.
Another way to use the firmware is to gather I/O samples and arrange for them to go wirelessly to an XBee and then to the serial port rather than the virtual wire mode.