It would seem that despite a 300% growth (from <4% to >12%) in PHP 5.X usage in 2006, it still only commands about 12% of the entire PHP user base. [...] As a 5.X release master, I am very interested in hearing what's holding back 5.X adoption and what can we, as the developers do in 2007 to help speed of 5.X adoption.

He notes that a lot of the concerns (speed issues, stability problems, etc) have already been addressed and are working like a champ in the latest PHP5 versions releases.

So, do you have a reason why you're still using PHP 4? If you're not using it, you know someone who is. Take this as an opportunity to give them a friendly shove in the PHP 5 direction.

You can also check out this response and opinion on the topic on cyberlot's blog as well as this post on the PHPWomen blog.