I'm not doing any PHP at my new job, so I have not really had to face this. But I have been following PHP development, as PHP is what I'm using for my experimentation and hobby coding.
From what I have seen, PHP 6 is not going to be a huge shift from PHP 5. Mostly 6 seems like a chance to kill all the stuff left over from 3 and 4 that has been depreciated and giving warnings in 4 and 5.
Right now I would recommend upgrading to 5 if you can. PHP 6 doesn't look like it will create any major coding issues unless you have too much old code sitting around. Of course if your being hosted then you are limited to what the host supports, and they are not fast to upgrade.