PHP 5.0.0 Beta 2 was released today. I started really playing with PHP5 last week and it has some really new cool features. XML support is vastly improved and now on a decent level (real W3C DOM conformance for example and SimpleXML for an easier way to access DOM trees) and the new stream wrapper stuff looks also very promising.
Get it, test it, play with it, report bugs and just have fun with it.