From the wikipedia entry on Dowsing:

Evidence
In a scientific study in Munich 1987-1988 by Hans-Dieter Betz and other scientists, 500 dowsers were initially tested for their "skill", and the experimenters selected the best 43 among them. These 43 were then tested the following way. On the ground floor of a two-story barn, water was pumped through a pipe. Before each test, the pipe was moved in a direction perpendicular to the water flow. On the upper floor, each dowser was asked to determine the position of the pipe. Over two years, the 43 dowsers performed 843 such tests. Of the 43 pre-selected and extensively tested candidates, at least 37 of them showed no dowsing ability. The results from the remaining 6 were said to be better than chance, resulting in the experimenters' conclusion that some dowsers "in particular tasks, showed an extraordinarily high rate of success, which can scarcely if at all be explained as due to chance ... a real core of dowser-phenomena can be regarded as empirically proven"[6]

Five years after the Munich study was published, scientist Jim T. Enright contended that these results are merely consistent with statistical fluctuations and do not demonstrate any real ability.[7] He noted that the best tester was on average 4 millimeters out of 10 meters closer to a mid-line guess, an advantage of 0.0004%. The study's authors responded[8] but Enright remains unconvinced.[9]

More recently, a study[10] was undertaken in Kassel, Germany, under the direction of the Gesellschaft zur Wissenschaftlichen Untersuchung von Parawissenschaften (GWUP) [Society for the Scientific Investigation of the Parasciences]. The three-day test of some 30 dowsers involved plastic pipes through which a large flow of water could be controlled and directed. The pipes were buried 50 centimeters under a level field. On the surface, the position of each pipe was marked with a colored stripe, so all the dowsers had to do was tell whether there was water running through the pipe. All the dowsers signed a statement agreeing this was a fair test of their abilities and that they expected a 100 percent success rate. However, the results were no better than what would have been expected by chance.

Some researchers have investigated possible physical or geophysical explanations for dowsing abilities. For example, Soviet geologists have made claims for the abilities of dowsers,[11] which are difficult to account for in terms of the reception of normal sensory cues. Some authors suggest that these abilities may be explained by postulating human sensitivity to small magnetic field gradient changes.[12][13][14]

One study concludes that dowsers "respond" to a 60 Hz electromagnetic field, but this response does not occur if the kidney area or head are shielded.[15]

A review of archaeological studies in Iowa suggests that dowsing is ineffective at finding unmarked human burials.[16]

Re: "Also, the L-Rods would be trained before the session (meaning in reality that the subconscious is trained) as to what the indication would be."

Yet, when I did it, I didn't even know that we were going to be "dowsing". My dad just said "get in the truck, we gotta drive near the marsh to do some work". He just put the rods in my hands and told me to walk back and forth accross the field. No training. In fact, I had no idea that I was "dowsing" (having never heard of it before). I figured this was just the way to find the water underground. Had no idea it wasn't supposed to work.