As mentioned by DynamicWebPages.de, the latest PHP usage statistics have been released for November 2006.

Overall, the spread of PHP seems to have settled down this month to a more stable value. For this month, the number of domain using PHP came in at 19 million (19,404,533) and the number of IPs using PHP is at 1.3 million (1,319,743). This drop can be somewhat attributed to several hosts being made aware of the "expose_php" setting in their installations and turning it off for security reasons.

You can check out this month's numbers as well as how it relates to previous months (and years) in this graph.