Just another great tutorial from ladyada.net
„If you have an Arduino project where the dev board is stuck inside a machine, or attached to the rafters or is inaccessable in some other way, a wireless programming/debugging link will save you tons of time. This tutorial is an extension on Rob’s version. In this tutorial, no extra firmware or hardware (other than a capacitor) is necessary. Just use the default bootloader. I use a ‚classic‘ Arduino but of course this can be easily adapted to any version or clone.“
http://ladyada.net/make/xbee/arduino.html