Python is not my native language so to speak, but Dan should be able to verify rain gauge type input interrupts in about five minutes with this tutorial and source code for the Pi.
http://raspi.tv/2013/how-to-use-interrupts-with-python-on-the-raspberry-pi-and-rpi-
Edit: There does seem to be more than a few open bug tickets for the RPi.GPIO library concerning interrupts. Perhaps a rain gauge is better suited for the AVR on the OSBee.
http://sourceforge.net/p/raspberry-gpio-python/tickets/search/?q=!status%3AVerified+%26%26+!status%3AInvalid+%26%26+!status%3ADuplicate+%26%26+!status%3ADone+%26%26+!status%3AWontFix+%26%26+!status%3AFixed