It's my feeling, whether the child is 1 week, 1 month, or 1 year - that they need their own space. I've seen children that have slept with their parents, and then when the parents decide it's time to end it, they have a screaming monster on their hands that actually CAN'T sleep by themselves. To me, the parents are just asking for a clingy child that can't function if they're out of sight of one or both parents, and it's reinforcing the child's need for the parents to physically be there in order to sleep.
I'm not saying that ALL children are this way, but I've seen way too many that are. I believe in doing what works for you, but if a new mother was asking my advice/opinion on the issue, I have to urge her not to do it - for the above reasons.