From Dotvoid.com, there's a new posting talking about the continued use of PHP4 by developers and hosting companies and how it's effecting the community as a whole.
It is true that the PHP 5 uptake has been slow. Many internet service providers as well as many developers are reluctant to switch. This is probably, as others have stated, a circular dependency.
As ISP:s are reluctant to upgrade the popular open source projects are hesitant to upgrade. I still think the wrong people are worried. Still PHP 5 is increasing it's share and finally the day will come when version 4 will be considered legacy. Hopefully before PHP 6 is here but not necessarily.