When posting the question I forgot that I can compile my own stats which for me would be even more relevant than some global "market share" data.
I'm selling a PHP script and I have the site urls of the people who buy or trial my software so I can easily do an automated "survey".
Currently the PHP versions supported by the script are 4.3+. The actual distribution of PHP versions, however, got me surprised (pleasantly). Out of 176 sites that I pinged, 7% were running PHP4, 3% - PHP 5.1 and the rest 90% - 5.2.4+, which I think is quite nice for anyone thinking about developing just for PHP5.
Of course, my data may be scewed - my users are mostly based in US and are more tech-savvy than the average. Someone with wider target market may have different results but I think the trend is good :)
Oh, and it's worth noting that from 176 sites, precisely 2 (two) were running Windows even though my software works just fine on it :)