It's closer than you might guess. According to Alexa statistics (which I know aren't perfect), WordPress.com is one of the top 20 web sites in the entire world, even though it is visited by "only" 4% to 5% of all Internet users each day [1], compared to Wikipedia's 10-15% [2]. The WordPress software has been around for almost a decade, and although most readers of WordPress blogs might not recall the name "WordPress", they probably have at least some passive recognition. Each hosted WordPress blog uses the "W" favicon by default.
Sure lots of people visit WordPress sites, but how many of them actually know who/what WordPress is and know what the logo looks like. I have to admit that if you'd asked me 5 minutes ago to describe the WordPress logo I would have drawn a blank.
[1]: http://www.alexa.com/siteinfo/wordpress.com
[2]: http://www.alexa.com/siteinfo/wikipedia.org