I'm not very scientific when it comes to training, but here is my 2 cents on it.
From my experience this is what I have found to be true, for me at least. I've trained others, and it works to some degree as well.
When you first start, as a beginner in lifting, you can get away with deadlifting more frequently. The reason being is the load that you are lifting. For example, if you have a lifter who is only pulling 200 lbs for his/her works sets the overall stress is going to be much lower than someone who is pulling in the 400s. Now, this is really relative in my opinion-- becasue work done to one individual is going to be entirely different for another.
The more you progress, the less frequency you'll need. More or less, this is based on the fact that your body will need more time to recover and you are not going to get results as quickly as when you are a novice lifter. I believe I read a thing by Matt Kroc many years ago where he explains this better... I'm sure you could find it with some searching around.
Since the deadlift is such a taxing exercise to the body and nervous system, consider the affects it has on other lifts. Primarily the squat. If you're fried from deadlifting your squat will suffer. However, most people who progress in squats will have a positive carry over to their deadlift. So, the idea is that you can squat, squat, and squat some more and be getting results in two lifts. You'll stay fresher, recover faster, and thus reap better results.
Personally, I can leave deadlifts totally out of my program for months, and come back and pull the same thing I did 3 months ago. I've even played around with deadlifting once per month, and have had great results with that. Likewise, deadlifting once a week works well to a certain degree for me, but I don't recover from deadlifts all that well, so I usually just play it by ear.
Another thing I have learned, is that ANY injury or serious strain I've had from deadlifting, has been the result of deadlifting too frequently. I have found that when I get tired, it is a fuckin horrible idea to push beyond that, because whether I realize it or not, my form fails just enough to where I can hurt myself pulling challenging weights. If I can attack a challenging session being fresh I stay healthier and can maintain proper form to pull heavy. I hope that made sense...
Remember, this is all done through proper programming and chosen the right assistance exercises to aid in overall strength development.