I use Dvorak almost exclusively (with US English enabled for when friends want to use the computer), but foolishly, I did not choose this keyboard layout when I ran through the initial setup. Every time I login, I must type my name and password in US English, before then being able to use Dvorak as I would like.
The solution is simple enough: run Setup Assistant again. Open Terminal (in /Applications/Utilities), and type:
sudo /System/Library/CoreServices/Setup Assistant.app\
/Contents/MacOS/Setup Assistant
Type your user password and then follow the onscreen instructions as you did once-upon-a-time. Reboot and enjoy using your preferred keyboard layout at the login prompt!
[robg adds: I ran the Setup Assistant, but didn't actually take it through to writing and rebooting, so I'm not sure if there are any downsides to this method.]