If I broke up with my boyfriend (more than 2 years) it would be really easy (physically) not to see him much. He works just a few minutes down the road, but lives about 20 minutes away and has lots of his own friends.
Emotionally it would be a lot harder, as his friends, have become my friends and my friends are his etc, and i would probably see him when i went out, which would be heaps bad if i saw him hooking up with some random chick (or someone i knew). I would also miss seeing his family.
I would try to at least be friendly to him, but like every one else has said, depends on why you broke up. If you just drifted apart, it would probably be much easier to be friends after you broke up. If he cheated, than that is a lot different.
It would also be weird as I only knew him for a week before we went out...so we werent "just friends" for very long at all.