In what namespace should you put interfaces relative to their implementors?

Specifically, when you create an interface/implementor pair, and there is no overriding organizational concern (such as the interface should go in a different assembly ie, as recommended by the s# architecture) do you have a default way of organizing them in your namespace/naming scheme?

This is obviously a more opinion based question but I think some people have thought about this more and we can all benefit from their conclusions.