Most of Iraq's population lives between Baghdad & al-Basrah, some 60-70% of Iraq's people in that area (counting Baghdad's population); the majority of the population in the most populated area is Shia (around 55-60% of Iraq's total population).
The so-called "Sunni Triangle" (a propaganda term invented by the occupation authorities and their media lackeys) has around 25-35% of the population, and many Shia live in the area as well. Contrary to the "everything is going great, nothing to see here people, hail Dear Leader W" line of propaganda put forth, the fighting is not limited to this area.
Baghdad was estimated to have around 5-6mil people (no telling if it's that much now), or around 1/5->1/4 of Iraq's people. Around 2mil of it Shia (the majority of them are assumed to be supporters of the anti-occupation leader Sayyid Muqtada al-Sadr), 1-2mil of the city is Kurdish.
this shows the breakdowns and has some population figures (remove the space between ".edu/" and "maps/" to view):--
http://www.lib.utexas.edu/ maps/middle_east_and_asia/iraq_ethno_2003.jpg
this one shows detailed population density, also has the populations of the 10 largest cities (again remove the same space to view):--
http://www.lib.utexas.edu/ maps/middle_east_and_asia/iraq_pop_2003.jpg
this has a listing of the populations of the major cities,
http://www.globalsecurity.org/military/world/iraq/city.htm