Amo Probos

The Yubikey is an authentication key suitable for use in multi-factor systems and is significantly cheaper and easier to work with than other hardware authentication keys.  The authentication server and several clients (including PAM) are available as Free Software.

The device presents itself to the host as a USB keyboard, and when you press (short or long -- it has two memory slots) the button (it's capacitive -- no moving parts) it "types" the authentication token.  This is very convenient, as long as your system and the Yubikey agree on the keyboard layout.

If you have a Dvorak keyboard, that is unlikely to be the case.  As of x.org 1.8 it is easy to use a Yubikey with a Dvorak keyboard by adding a section to your xorg.conf.  Of course, you might not have an xorg.conf anymore since almost everything is autodetected, but it's still supported for those exceptional cases like this.  The following instructs X to use the "basic" keyboard variant ("dvorak" is a variant) when the Yubikey is inserted.

Section "InputClass"
  Identifier      "yubikey"
  MatchIsKeyboard "on"
  MatchVendor     "Yubico"
  MatchProduct    "Yubico Yubikey II"
  Driver          "evdev"
  Option          "XkbRules" "evdev"
  Option          "XkbModel" "pc105"
  Option          "XkbLayout" "us"
  Option          "XkbVariant" "basic"
EndSection

Update: Just don't leave it plugged in when X first starts.