Re: the "nag page" idea... I'd be curious to see some opportunity for user feedback on that page. Maybe that's a good way to get some more solid information regarding percentages of people still on PHP4?

I'm one of those people still currently using PHP4.4.4. I have no control over my web host's decisions regarding what they update and when. So if WP moves to PHP5-only, that means i either don't upgrade for an indeterminate amount of time, or i find other software to use. I don't like the idea of being given an ultimatum based on something over which i ultimately have no control - and that alone may make me decide to start using other software.