I spend a lot of my time at work using PHP, and I don’t see any of Rasmus’s proposed changes breaking any of my existing apps. I don’t use register_globals, magic_quotes_*, safe_mode, or deprecated functions.
The only problem is: will anyone switch? Large hosting providers probably won’t. I still use PHP 4 just because Linux distros have been so slow at moving to PHP 5.
At any rate, I hope Rasmus gets his way as all of his proposed changes sound excellent to me!