An important consideration is protein utilization/digestion/assimilation.

I think that properly chewed, animal-source protein, run through a healthy digestive tract can allow the body to make good use of far less protein than the recommended 1-1.5gms/lb dose.

Glutamine and BCAA (taken on an empty stomach between meals) as previously noted by Jon can also allow you to take in less "protein" and still gain strength/stimulate hypertrophy.

It will vary for each person. It would be the amount of protein that you can maintain your body composition.

I'd say, if you were going to try and maximize your anabolic potential, to make sure your testosterone and growth hormone needs are well-taken care of. Also, branched chain amino acids and glutamine should probably be emphasized. The BCAAs will help prevent muscle catabolism.

I think the whole nitrogen balance thing is kinda bunk. I'd like to see some legit papers on this. I remember reading all about this when I first started getting into weightlifting, 15 years ago, but I haven't seen it make a real difference in reality. I also read, back then, you could only assimilate 30g of protein or something similarly low per meal. Since eating Paleo, I put down much more than this in a meal and feel my muscle anabolism is still just as good.

Like all things, individual variance is quite high. For most people, having a large caloric content from amino acids will be beneficial to gaining muscle. It sounds like some self experimentation may be appropriate. See if you can get away with consuming less protein and still build muscle.

Arnold is certainly a genetic freak, and more incline to build hypertrophy than most people, but he is no scientist.

You would only need 100g of protein when you are that jacked up on roids!

I haven't read his book, and he may discuss what other "supplements" he used to build muscle, a few of which were probably not considered Paleo. Maybe extra protein isn't needed if you make other choices.

No one really knows for sure. Why don't you experiment with different ratios and see which one gives you best results? A side note, muscle building a la Arnold and health have very little in common.

The best response I've ever read on this subject comes from an interview with A. Scott Connelly, who probably knows as much about dietary protein and body composition as anyone alive:

"Q: In your opinion, how much protein should a serious strength-power athlete consume on a daily basis?

A: My real world answer to this question is to “consume as much as you can.” The response in muscle growth induced by resistance training to increasing dietary protein intake is virtually linear over a range of intakes that are attainable by human beings. For example human research shows that muscle mass gains are doubled by escalating intake of protein from two to three grams per kilogram of bodyweight. I have worked with subjects who’ve reached daily intakes of 400 grams per day with spectacular gains in LBM [lean body mass].

There are theoretical concerns surrounding the fact that any excess intake of amino acids over immediate requirements activate degradation pathways and that there can be finite limits to these mechanisms. In reality this is only an issue for the rare individual with genetic disorders of amino acid metabolism (phenylketonuria, for example) as the capacity for amino acid oxidation [burning] can be ramped up very rapidly to keep pace with intake values that are practically attainable. The pattern of consumption should be multiple feedings throughout the day with the protein content of each feeding representing equal fractions of the daily total. Disproportionate large quantities consumed at single feedings increases oxidative loss of amino acids as the system is geared to maintain individual plasma [blood] amino acid levels within certain limits of one another. Flooding the plasma with large excesses above physiologic ranges is immediately followed by irreversible oxidation of the excess."