Here's the pic:

And the link: http://cowjam.net

At the moment I'm using a pocket digital camera (an olympus mju 8000), which has a maximum shutter time of four seconds.  I open the shutter then set everything going - the speed of the flash dictates the length of exposure.

I'm doing all this in the cupboard under the stairs as there are no windows.  The process is - half press the camera button to get it to focus, keep my finger half pressed (or it'll change focus).  Turn the light out.  Press Go on the arduino.  Fully press the camera button.  Drop something.

With a bit of luck I'll drop whatever it is through the beam (not that easy in the dark).  It's quite frustrating getting a good shot - my aim has to be right, I have a small window of time to get it done in and the autofocus has to be right and I have to have the camera aimed at the right section of the water.

I've changed the way the flash is fired now - once the the laser is shining it takes a light reading and fires the flash when the light level changes from that (with a sensitivity variable built in as I wasn't sure how sensitive it needed to be).

The next step is to put it in a box, rig the sensor and laser up so they're stable and build some kind of frame for dropping drops/marbles/whatever so they reliably go through the beam.
Once I'm sure it's all sound and reliable I'll get my old SLR out, but I don't want to waste film getting crap shots like I am at the moment.
I can manually focus the SLR, set the aperture and lock the shutter open, so it should be a little more relaxed then.

Sam

If your camera has an IR sensor or remote, you could trigger it that way by flashing an IR light at the same time as the flash.  
I'm sure you have already herd of Doc Edgerton, but if not, check him out too.
http://en.wikipedia.org/wiki/Harold_Eugene_Edgerton

bld, I've contemplated using a servo for the physical remote trigger on the SLR (it's not digital, it's an Olympus OM2).

I might get another light - I don't really fancy dabbling with mains - it's a good idea though.

Very tempting bld.  Will those components work with 250v 50Hz? I'm British.

Can't seem to that in a British shop.  I'll have to call in to my local electronics store, I've little idea what I'm doing yet.

luidr, having the laser/sensor just above the surface of the water (about 1cm) is plenty fast enough.  I've had to add a small delay to get anything other than the object just touching the water

The video is a good idea.