Symfony2 and Lithium are two frameworks I can currently think of that use them extensively. On the one hand, Symfony2 has a quite extreme implementation, and on the other, Lithium's is much more concise. The backslashes can be used to represent a hierarchy.

Double colon is already being used for static member referencing, and in a few talks I've heard from Rasmus, they weren't able to find a way to overload it for Namespaces.

Another option that was discussed was a triple colon operator, which most were opposed to b/c of it's length. I personally think the backslash is ugly, and I kind of like the triple colon operator better b/c of it's similarity to the double colon operator. Regardless of which one is prettier, I wish the double colon operator could have been overloaded.