@Dougal, You're right, I forgot that WordPress phones home with everyone's server information (unless you know how to unhook the action, there is no opt-out or opt-in or warning about privacy). So they do indeed know 99% of everyone's PHP info, their server version and IP, what plugins they use and other info.

My PHP version statistics are not made up and not out of thin air, I really did survey that many sites for bbPress back in April (and will include thousands more in October). However I did glance at them wrong in my rush to post and did not add up the different 5.x versions properly, so the PHP 4 use is closer to 25% than 50%, but most certainly higher than 15%. I'll post the exact numbers of what I find in October on bbPress.org.

The WordPress community is NOT just made up of sophisticated users on dedicated or VPS servers who know what they are doing and how to keep up with PHP. There is a far greater number of average users on shared hosting who do not have control over their PHP (nor want to care about it, they just want to blog).

I don't think PHP 4 should be supported "forever" or past next year. But another year isn't going to kill WordPress development. There is nothing so wonderful in PHP 5 that can't be easily emulated in PHP 4. I have no problem with a nag in 2.9 (and 2.9.1 and 2.9.2 and 2.9.3 lol).