If you have an existing Arduino, you can use it as an ISP programmer to program the OpenSprinkler. Here are the instructions of how:
http://arduino.cc/en/Tutorial/ArduinoISP
Although it will be quite tricky because you will need to layout the wires yourself. So it’s probably still easier to get a DIY programmer from my website.
You cannot plug in the OpenSprinkler’s microcontroller to your Arduino and program it that way. The reason is that Arduino itself does not have an ISP programmer — it only has a FTDI chip on board, hence it requires the chip to have a bootloader which takes too much space for the latest OpenSprinkler firmware.
The difference of an ISP (in-system programming) programmer is that it directly writes to the microcontroller’s flash without any bootloader. Therefore it takes zero bootloader space.