Almost 3 years ago we released a version of WordPress (3.0) that allowed you to pick a custom username on installation, which largely ended people using “admin” as their default username. Right now there’s a botnet going around all of the WordPresses it can find trying to login with the “admin” username and a bunch of common passwords, and it has turned into a news story (especially from companies that sell “solutions” to the problem).
Here’s what I would recommend: If you still use “admin” as a username on your blog, change it, use a strong password, if you’re on WP.com turn on two-factor authentication, and of course make sure you’re up-to-date on the latest version of WordPress. Do this and you’ll be ahead of 99% of sites out there and probably never have a problem. Most other advice isn’t great — supposedly this botnet has over 90,000 IP addresses, so an IP limiting or login throttling plugin isn’t going to be great (they could try from a different IP a second for 24 hours).