I was with a mixed Shia/Sunni unit when I was an advisor. They worked well enough together, but the distrust that was out there was very much what Michael Shannon posted. The Sunni often described the Shia as ignorant hillbillys. When you asked them why they felt this, and how the Shia came to be that way, especially when presented with a Shia guy in the unit who was neither ignorant, nor a hillbilly; the Sunni would actually reflect and reply that they are uneducated, and often they would have to agree, when pressed, that Shia lack of education had more to do with govenrment policies than anything specific to the Shia. The second bullet is the true cause of Sunni misttrust of the Shia: Iran. Many, many Sunni that I knew would refer to the Persians, and they would manifest this by pointing out that SCIRI and Al-Sadr had Iranian backing. The Sunni's knew that the Shia's were Arabs, and that the Shia had fought the Iranians in the Iran/Iraq war, but they were concerned that the Iranians have too much influence of the body politic. Finally, there is the Sunni worldview issue. Under Saddam the Sunnis were actually led to believe that they were the majority. Based on the Sunni arrogance towards the Shia, they tend to underestimate tha capability of the shia because of their stereotyping or their belief that there are actually more Sunni Arabs in Iraq.

The key point of friciton is the Sunni's mistrust of Iranian influence over the Shia verus the Shia mistrust of the Sunnis based on how Saddam's (Baath) party treated them in the past.