FWIW The boxen I get my mail on is running PHP4 and I have absolutely no trouble with unicode support in my mail (to or from).
The unicode changes in PHP6 are a little more complicated than that and change how most of the engine works, this is a good thing IMO as native unicode support is vital to support the growing needs of PHP applications to support a wide range of languages.. although .... I guess i'm not looking forward to reading php code that says <?php function 北方话/北方��() { echo "Hello world "; } :-D Regards Marco