Common sense, simple security for WordPress. Many presentations have lots of complicated .htaccess tricks, moving/hiding files, etc. However, if people are overwhelmed with details, they tend to not do anything. If I were to summarize what you MUST do for security, I'd say:
1 - BACKUP - find a backup tool and use it. Subscribe to VaultPress.com or host your site with WPEngine.com or purchase BackupBuddy plugin and schedule regular backups. If you're short on cash, use BackWPUp plugin and download your wp-content folder.
2 - UPDATE - All plugins, themes, and WordPress at least once a month or whenever there is a security update. Sign up for an account at WordPress.org, so you'll get notices of WordPress security updates.
3 - DELETE -- All unused plugins and themes. These are your biggest security risks. Delete all unused copies of WordPress you might have installed on your server.
4 - BE CAUTIOUS - Don't use plugins willy nilly. Do some research. They are not all made the same, and they will leave you vulnerable to hacking.
5 - PASSWORDS -- Use strong, randomly generated passwords, all different, for everything - your hosting, ftp, WP login, and email. Use 1Password.com to track your passwords easily and securely.
6 - SECURITY PLUGINS -- Run Firewall 2 and Limit Login Attempts. There are others, but I don't know how well they play with others and what things they modify. You can check out Bulletproof Security and Better WP Security.
7 - BEST PRACTICES - See the slideshow for some other best practices regarding users, comments, etc.
If you just do the above 6 things systematically, you'll be far ahead of your peers! Good luck!