There seems to be a growing amount of interest within the PHP community about the slow rate of PHP 5's adoption. Call me crazy, but I tend to think of this as a testament of PHP's success, because it might mean:

• PHP 4 suits many people's needs, so there's no pressing reason for them to upgrade (think about Apache 1.3).
• PHP is being used in environments where upgrading is a very controlled process (Yahoo, for example).

I also think PHP 5 will reach a tipping point (check out the book) eventually, so the early rate of adoption isn't necessarily a forecast of PHP 5's future.

Speaking of PHP 5, Upgrading to PHP 5 is one of my favorite PHP books. Despite being a technical reviewer for the book, I find myself referencing it more than any other. Adam does a great job of explaining the new features in PHP 5, and the chapter on OOP is especially good.