There's a new book/DVD out called 'The RAVE diet and lifestyle" (most of the info in the book is in the DVD and vice versa--you can get them both for $19.95 from
www.ravediet.com ). In it, the author talks about the benefits of a vegan diet, and dietary myths, such as the protein myth. The World Health Organization recommends 5% of calories from protein (you read correctly,just 5%), the same percentage that is in human milk (which furnishes babies needs at a time when they are growing the most). Even taking into account that other forms of protein might not be quite the exact "fit" that human milk is, and doubling that requirement, it brings you to 10%. Let's say that someone is in need of extra protein to build and repair tissues, like after getting hit by a bus and being mangled. Double that again. That only brings you to 20%.
It's also quite interesting that, vegan, vegetarian or omnivore, it would be difficult to eat as much as 40% protein using REAL FOODS. Some kind of supplementary protein source would have to be added. I have enough faith in Mother Nature (or whatever powers that be) to believe that we wouldn't be created to need more protein than we can get in a natural diet.
40% is very tough on your body, though, as Carole said, plant protein is less damaging (because of fewer sulphur-containing, acid-producing amino acids). It can weaken the immune system (as per Andrew Weil, who is rather conservative in his views). And it's not necessary, so the excess is either excreted (and since protein is the most expensive nutrient, it's a shame to send it down the drain...if you know what I mean!)
I think it's quite interesting that these newer "high protein" diets came out at about the time when people started eating more fruits and veggies and fewer animal products. Not a coincident, IMO. And if you consult any reputable health organization, you'll see that they do not at all endorse these diets.
Carbohydrates are protein sparing, so by eating a sufficient amount of natural carbs (not processed), you can make sure the protein you eat goes to muscle building rather than being used--quite inefficiently---for glycogen production.
I feel that as long as you are eating a primarily whole-foods diet, and a variety of foods, you will be fine. It's pretty difficult to have a "protein deficiency" unless you're living on twinkies and cola or just citrus fruits.