My parents hardly ever fought, but we were 15, 14 and 11 when they split up and we weren't stupid. When Daddy sleeps on the couch for 8 months, you know that everything isn't kosher. About 2 months before my dad moved out, he sat us down and told us that him and mom were going to be splitting up and he was going to be moving out. It wasn't our fault and blah blah blah. Since I know my dad and his behavior, I wasn't surprised. I was actually pretty pleased that the tension in the house would be gone. This part was handled fairly well and we were older, so it wasn't that big of a deal.
When he actually moved out, I was on my way home from a trip to Vermont. I called him in New York and talked to him about picking me up from school when the bus arrived. He didn't say a word about it, but he was packing at the time. When my mom picked me up to take me home, she told me that Dad had left and I walked into a house of crying family members. That sucked and I don't reccommend that course of action to anyone.
One thing I learned throughout the whole thing is this: If you have a son, make sure he understands that just because his dad is gone, he does not need to assume the "Man of the House" role. It was really draining for my brother (14 at the time) because he felt like he had to fill my dad's shoes.