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. He published a 2007 paper entitled 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.