From memory, I think the WordPress plug-in still permits password authentication even when you configure it with a YubiKey. So if you lost the YubiKey, you just need to remember your password to get back in. That said, worst case is you can move the plug-in directory to disable it, then in your database:
Code:
SELECT user_id FROM wp_users WHERE user_login='your_login';
DELETE FROM wp_usermeta WHERE user_id=your_id AND meta_key LIKE 'yubikey%';
I set up the YubiKey plug-in on my blog initially, but then when I got OpenID going with two-factor authentication, I decided to disable YubiKey authentication on my user login and use OpenID instead. Rather, I use password authentication when I'm at home (and thus, only 100Mbps Ethernet between me and server), and the YubiKey via OpenID when I'm outside the home network.