Hey Andy,
Laser range finding can be extremely difficult, or simplistic, depending on required accuracy and measurement frequency.
I believe there are three basic methods.. One relies on a device that can measure the incoming angle of the reflected laser beam. I am not too sure about this one, but I believe it is quite simple.
The two other methods are build on the principal that light takes time to travel. A laser beam is modulated with a sinewave and sent out of the unit. Before it is sent, a small portion of the beam is sent to a reference photodiode through a 90deg beam splitter. Then the beam is returned through a large lens and focused on another photodiode. The two signals after transimpedance amplification are compared for phase differences, then the resultant is calculated. I forget the equations, but it isn't a hard conversion. The problem with this is the modulations and the components required.
Another method, which I would consider if I had to do it, involves sending out a laser pulse. Once this is sent out, it triggers a counter to start counting at a high frequency. Then once it is recieved, the counter stops. This method is fairly simple, but involves a lot of highspeed components and fine tuning to perfect. You would need to use a FPGA for optimal results!
The optics involved in some of these systems are expensive... If you don't need good pinpoint accuracy, large distance measurement, good accuracy, then consider using one based on ultrasonics. You can use the same principals as the last method described, but you do not need a very fast counter.
To be aware of.. Both systems have weaknesses.. The laser light needs to be reflected back and detected.. so if you do not have very sensitive photodiodes or a medium that absorbs the laser or transmits it, then you are in a lot of trouble. Also, sound absorbing materials will not be detected with the ultrasonic method.
Steve