WordPress.tv has an interesting video by Chris Wiegman of bit51 entitled Securing WordPress is Easier Than Making Coffee. If you have a WordPress site this is worth an hour of your time. He begins by showing how a seemingly trivial plugin can open you up to an easy exploit requiring no more than Chrome. Wiegman says that the core of WordPress is actually pretty secure but that poorly written themes and plugins can make you vulnerable.
I particularly liked the section about tools. Wiegman discusses several tools that you can use to check your site, back it up, and harden it against exploits. Many of these are free so even the casual blogger can afford to be protected.
Again, there's a lot of useful information in the video and it's worth watching.