I designed a good high speed one that uses either A) a laser, B) a sound sensor, or C) a contact switch and it worked wonderful (I did not takes shots with the arduino one yet, but I tested it... so I dont know about lag (should not be a problem, but it does allow for controlled timing)
well, I cant find the code at the moment, but here are the basics: setup sets pin 13 (LED) as output (it is the ready light), and pin 2 as a output (it triggers the SCR for the flash unit), pin 4 as an input, and it waits for the pin to close with ground to trigger the SCR on pin 2 [this is the contact switch], pin 7 is also an input connected to a photo diode, so when a laser hits it the pin can conduct to ground, and when the laser beam is broken it does not conduct (or, you can use a transistor or relay and WAIT for the pin to be able to flow to ground), pins 8 and 12 are connected to normally open buttons, allowing for delay settings (when connected to the computer, it would show on the serial output how many microseconds it would last)... then lastly is an analog input that uses a microphone [or piezo unit, like my preference] to trigger at a certain threshold...
oh well, the above is almost meaningless until I find the code... I used a flash drive to store it, and I left it at school somewhere : - (
but hey, nothing private was on it 
also, it is possible to get results immediately for high speed if instant film is used, and you can apparently get high grade (and yes, I know Polaroid does not make it anymore)... but it is hard to find a moderately priced digital that can respond fast enough.