It's a very interesting question...
From a historical perspective, we know that "male bonding" has existed, in some form or another, since the beginning of time (or at least the beginning of recorded history). In the artwork and literature of virtually every civilization that we have discovered, the virtues of male-male relationships (not necessarily homoerotic or sexual relationships, although a good deal of them were) are extolled, expounded upon, even idealized/idolized. The ancient Greeks, for example, believed strongly in men cultivating intense relationships with other men... so much so that young men were often pressured into having sex with either their peers or, more commonly, with respected older men of the society.
Along that line, it is quite interesting to me that couple of you have mentioned "Platonic" relationships. In ancient Greece, sexual tension between the student and the teacher (who were both always male) was viewed not only as acceptable, but as a necessary component of the educational system. It was Plato who first explored this concept in his writing; he came to the conclusion that the highest ideal of any teacher and student should be to be consumed with sexual desire for one another--to be consumed, but not to act upon it. With the desires thus frustrated, Plato believed that the sexual energies of both teacher and student could be diverted into their studies and lessons, thus strengthening whatever was being taught. It is fascinating to me how the meaning of a Platonic relationship (and yes, it does get its name from Plato) has changed from the thwarted sexual desire of a teacher and pupil to an absence of sexual desire between two friends. But I digress.
Although the Greeks are perhaps the most well-documented on the subject of male bonding, they are by no means the only one. Numerous cultures, both ancient and modern, abound with examples. If anything, the rampant homophobia of the last hundred or so years has done much to discourage what seems to be the natural, normal act of forming intense emotional connections between two male friends.
And I completely agree with HungJon--physical intimacy is key. We can't escape our primate nature, after all. If you think about it, I'm sure most of you will realize that men will always find some way to touch one another--if not in open displays of affection (e.g. hugs, physical proximity), then in veiled ways--rough-housing, horse-play, monkey-business--call it what you will. The truth will out. I mean, why do you think the ancient Greeks were so fond of wrestling?