As far as I'm aware there's nothing in PHP 6 that can't be supported from PHP 5.2.1 up, but there's plenty in PHP 5.3 that a lot of developers will like.
AND SHOULD BE BEING DEVELOPED IN PHP6 SO WE CAN AT LEAST GET A CLEAN UNICODE STABLE SYSTEM AVAILABLE TODAY !!!!!!
Please don't shout, thanks, I'm nursing a headache.
Most of it _was_ developed in CVS HEAD, and then back-ported to 5_3 when it became clear that Unicode support is likely to take a while. Extensions don't tend to be developed that way (ever), because the PECL system is such that whatever's in CVS HEAD needs to work with the current release. But that's all changing too.
- Steph