On the left side of the Canon EOS 450D (and probably on any other EOS camera), there's a connector for a remote shutter trigger. You can make your own remote trigger by sticking one side of a 2.5" jack-jack cable into the connector, while bridging two of the rings on the other side of the cable.
By connecting rings 1 + 3, you can focus and trigger the shutter; connecting rings 2 + 3 triggers autofocus only. With that information, you can easily make yourself your own remote trigger, maybe even an automatic trigger using the arduino and some sensors.
For high speed photography using the arduino and an external flash, i'd recommend the approaches by [nosmos.org] and [glacialwanderer.com].