In WordPress' Terms of Service, they say:
"By submitting Content to Automattic for inclusion on your Website, you grant Automattic a world-wide, royalty-free, and non-exclusive license to reproduce, modify, adapt and publish the Content solely for the purpose of displaying, distributing and promoting your blog."
So they can freely use your content to promote WordPress. Great. But do I actually own what I write, or does WordPress? Anybody know?