Matt Mullenweg did a little research study for himself, based on his own “guilty pleasure” for reading Techmeme. Techmeme's leaderboard, for those that aren't aware, is basically the benchmark all tech sites measure their influence by.
Matt analyzed which of the Techmeme 100 run WordPress, and found that 43% of them do. Based on the spreadsheet Matt created, 14 sites run WordPress VIP versus 27 on non WordPress.com solutions (Note: this adds to only 41 sites, because I write this on a different day than Matt's post – the list varies slightly day to day).
I honestly would expect a bit higher percentage of WordPress VIP than that, but it's cool that even in such a high-profile list of folks that could afford pretty much any CMS, WordPress still dominates.