At this stage it would be not certain as to which countries will compete. That will only become clear when the 2012 Olympics come closer. A lot can happen in the next 4 years that could determine who will attend them. Answer #2 204 countries are participating in 2012 summer Olympics.
There are 202 countries in the world and in Athens 2004, all 202 countries entered teams for the first time ever.Men and wemen are allowed to compete in all the events that are performed during the olympics.
205 countries participated in the 2012 London Olympics according to the official London Olympics web site with a total of 10,961 athletes. See related links below for the list of countries and number of athletes from each country.