I found this in the source code for pulseIn():
"/* Measures the length (in microseconds) of a pulse on the pin; state is HIGH
* or LOW, the type of pulse to measure. Works on pulses from 2-3 microseconds
* to 3 minutes in length, but must be called at least a few dozen microseconds
* before the start of the pulse. */"
However, this is *not* in the documentation here: http://arduino.cc/en/Reference/PulseIn
Please update the documentation to contain the above information, which is critical for certain applications. The line I especially found useful from the source code is that the function "must be called at least a few dozen microseconds before the start of the pulse."