PHP 6 is a ways off. PHP 5 is the "now" item. What you write in php 5 will work just fine in php 6 (just like php 4 scripts worked fine in php 5). The difference comes down to security of the installation. Many php 3 and php 4 scripts relied on things like "register globals" being on, and php 5 defaulted that to "off" and php 6 removes it all together (from what I remember reading).
The best way to keep your scripts from failing is to develop in the latest stable release (5.2.5 at the time of this writing) and code to more "strict" standards. Then later down the line when 6 is about to be released, or before it goes mainstream (i.e. 6.0.x, 6.1.x even) set up a test environment and run your scripts in there.
I'm not 100% sure you fully grasp how the change in php works. It's not like a kernel or OS update which is a quick and easy thing for the end-user. Instead most hosts will test the next php version some time after the initial release to see its viability. When php 5 came out, almost no-one adopted it; and so it stayed for several years (I wanna say about 2 or 3). It wasn't until recently (within the last year or so) that php 5 really got a huge push and PHP decided to drop support for php 4. So you'll have plenty of time to check your scripts in php 6 before it goes "live" to everyone. Plus, if you have a dedicated server, you can run both php 5 and php 6 concurrently. You'd just have to define a new extension for older php versions (like .php5).
Hope that helps.
