Who is Satoshi Nakamoto? I have heard about him and read his paper.
Why is he no-longer publicly involved in Bitcoin?
Satoshi Nakamoto is the elusive and private creator of bitcoin. In October 2008 he published the paper Bitcoin: A Peer-to-Peer Electronic Cash System (pdf warning) which is the basis of the bitcoin client/protocol. He has stated his nationality as Japanese and published a PGP key, has a sourceforge page and aside from that not much is known. It sort of follows that someone interested in creating a distributed pseudonymous online currency would be a very private person.
It is also suspected that Satoshi may be a pseudonym for any of a number of people involved in the original development - a mask used for safety and protection against legal persecution for the creation of such a potentially disruptive product. To that end, Satoshi may still be involved in bitcoin, although as you stated, not publicly. As with everything else about Satoshi, the reason for his lack of current involvement is unknown.
The two of the most important clues we have are:
The two non-clues we have are:
The reason these are non-clues are that anybody can use any language or dialect of a language, and anybody can invent any pseudonym. So most likely Satoshi wants that we believe he's British or Japanese in an attempt to conceal the identity.
Also there is some speculation based on Satoshi's code that Satoshi is not necessarily a single person but could rather be a team. This agrees excellently with clue (2), since a large team might have less incentives to convert the million bitcoins to something of an inherent value than a single person would. But it would have to be a pretty large team, though (otherwise the bitcoins in there per person would favor converting that to real money), and every member of that team would need to keep that same secret. Most plausible is that the team is in fact a state actor, since something that once was 64 billion dollars wouldn't be unused even if the team would be a large company for example. As an example, if we assume Satoshi is Apple (the most valuable company in the world), that company could have paid
an extra dividend for one year (edit: an extra dividend for over 4 years, as Apple seems to pay less than 15 billion/year in dividends) by converting the bitcoins to cash. So claiming that Satoshi was Apple doesn't make sense. In a civilized world people working for a company wouldn't hide the existence of something that was once worth 64 billion dollars.
This leaves two most plausible explanations, the largest state actors in American time zones:
Considering that United States has done a lot to improve encryption (and in some cases embedding backdoors to encryption algorithms), this is by far the most plausible explanation. 1 million bitcoins is pocket cash for United States. However, this theory would mean there's some reason for the state actor to conceal the identity of Satoshi, and we can't know (we can only speculate) why United States for example would create bitcoin in secret. At least the United States intelligence services seem to be capable of keeping information secret at least to some extent -- true, there has been an Edward Snowden but only one Edward Snowden.
But it wouldn't be impossible for Canada to be involved, however; after all, Canada is part of the Five Eyes alliance with United States and many other countries.
Personally, I think it would be an excellent idea for for example NSA to create a cryptocurrency that the majority of the world's population consider more private than cash, but that actually is far more public than for example any bank transaction. The claim that Satoshi = NSA is entire speculation, though, but the incentive is there. If you want to access all Internet traffic in the world (presumably to identify something illegal from it), doing something that might give you a view to a large portion of illegal financial transactions would be very useful. After all, those contain most of the interesting transactions, the ones you want to access anyway (plus a lot of "investors" wanting to be rich by buying Bitcoin and then end up losing a lot of money on that speculation).
Also, an observation: Tor has been created by United States, but publicly. Wouldn't be impossible for Bitcoin also to be created by United States, but this time privately.
But that starts to be more speculation and less facts, so I'll end the wild theories here.