From Lukas Smith's blog:

So today we had an IRC meeting to discuss namespaces in PHP. We came to a final decision replacing the current double colon separator with a backslash to solve ambiguity issues with the current syntax. So far there has not been much of a reaction on list, except for a post which mentioned concerns related to the fact that backslash is currently used for escaping, but which was quickly responded by Greg to point out that yes we were aware of this, but it is not an issue if you know how to use the backslash properly in double quote strings.

You can find out more about this implementation using the backslash on this entry in the PHP.net wiki (near the bottom of the page).