SEO egghead Jamie Sirovich just posted about his new WordPress Firewall Plugin. Getting hacked creates a lot of work, so using a firewall (any firewall) is going to save you a lot of headaches down the road - especially if you're working with multiple blogs or managing WP installs for others. The plugin "investigates web requests with simple WordPress specific hueristics to identify and stop most obvious attacks."
While it may not stop the more sophisticated hackers from gaining access, something is most certainly better than nothing.
In addition to blocking common attacks, other features let you set up an automatic email to notify that a potential attack has been detected and blocked (with the offending IP and parameters) and let you whitelist your IP (as well as a page, a variable in a page or a variable on every page).
The WordPress Firewall plugin can be downloaded here.
Sample of the email sent when an attack occurs and is blocked: