I found a recent report putting WordPress at 10-15% of the Alexa Top 1 Million Sites.
Do you have any references to other similar data or reports (not only Top 1M)?
Most of the CMS usage stats originate from Built With.
I've put together a collection of various reports around the web.
WordPress.com (Does not include self hosted)
BuiltWith WordPress Usage Trends
CMS Distribution Stats
CMS Distribution Comparison