In the past I have always ignored the "Hodor = The Great Other" chatter because I thought it was some sort of joke referring to the actual character Hodor. I didn't realize that it meant that "Hodor" was the actual name of the Great Other
But with some new recent discussion on reddit about it I gave it a chance and now I'm like
To those unfamiliar with the theory, it stems from the fact that in Norse mythology the god of winter has a similar name:
Melisandre says about the Great Other (the counterpart to R'hollor) "the name that must never be spoken"
The theory is that the character Hodor (real name Walder) stumbled upon something in the Winterfell crypts, that thing possibly being an avatar of the Great Other that is locked in the crypts via blood magic. Walder was so destroyed by this incident that he can only speak the name of the being that scarred him.
This theory would explain the cause of the character Hodor's oddity, it would explain the repeated saying "there must always be a Stark in Winterfell" (due to the blood magic of locking up the Great Other), and it would explain the castle name Winterfell (the place where Winter fell that ended the long night and the Great Other was defeated).
There is more to it, but the reason this theory is being reinvigorated is because GRRM appears to have said that we will learn of the character Hodor's reason for only being able to say Hodor:
http://grrm.livejournal.com/420202.html?thread=21152618#t21152618
You can read more in depth about the theory with good discussion here:
http://www.reddit.com/r/asoiaf/comments/32nlpv/spoilers_all_the_importance_of_winterfell_and_a/
and here
http://www.reddit.com/r/asoiaf/comments/32p8bz/spoilers_all_grrm_says_we_will_know_the_cause_of/