I've noticed quite a few web hosts provide both PHP4 and PHP5 in their hosting packages but that they default to running in PHP4 unless you you specify it to use PHP5 in your control panel or .htaccess file, for example with 1&1 you have to add:

AddType x-mapp-php5 .php

So, I guess if some hosts are running PHP4 as default, this might account for the high percentage of PHP4 WordPress installs.

I wonder with the "nag page" idea wether people should be reminded that they may be able to configure their web hosted to run PHP5, and that is is often a very easy process.