Since a few hours we're having, thanks to Edin, Windows snapshots for the upcoming PHP 5.3 release. In combination with the latest sources from CVS everybody should be able to test the current state.
There are quite a few new features already committed to the CVS tree, some of them are documented in the NEWS file and READMEs in CVS, most of them have been commented on Planet PHP so I suggest searching there for more information and keep this list short:
Namespaces: A way to help you organizing your code.
Late Static Binding: Gives you the class name used when calling a static method.
Improved ini-handling: .htaccess file like per-directory configuration and much more
__callStatic magic method: similar to __call but for static calls
mysqlnd: a replacement for libmysql for better PHP-MySQL-Integration
getopt() on any platform (inclding windows)
As you can see there's lots of stuff which needs testing! The earlier bugs are found the earlier they can be fixed! In case you ever wondered how you could give back something to PHP (which you certainly should do) you might take a look at our test coverage reports to see which areas of PHP need more testing and provide additional tests. (See qa.php.net and Marcus's slides for an introduction to test writing) Having more tests is important for us - and you don't have to know much about PHP internals to write proper tests (sometimes it's even better to write black-box tests...)
But well, there's not only PHP 5.3: If you're interested in unicode you should certainly take a look at PHP 6 and if you're running older systems you should consider upgrading to the latest 5.2 release instead of waiting for PHP 5.3 - the stable release is not expected within this year and many things there can change till it's released.
And as a final note: Test the snaps as often as you can - generic tests can never cover all use-cases and - as said above - fixing regressions is harder after a release than during development. (And be warned: We won't take any complaints from people not testing in time)