There's a new this php-dev mailing list item posted today from Andrei Zmievski of Zend with a full outline of the Unicode support that's coming for PHP.

The basic goal of this document is to describe how PHP 6 will support the Unicode Standard natively. Since the full implementation of the Unicode Standard is very involved, the idea is to use the already existing, well-tested, full-featured, and freely available ICU (International Components for Unicode) library. This will allow us to concentrate on the details of PHP integration and speed up the implementation.

The post goes on at length about things like the "Implementation Approach", different types of encoding (Internal, Fallback, Runtime, Output, HTTP Input, and Script), and some of the "Conversion Semantics".