I prefer having tap-to-click enabled on my MacBook Pro. However, it's always been annoying to me that when I was at the login screen, I had to use the trackpad button. I asked one of the Geniuses at the local Mac store, and he told me it wasn't possible. Not believing that it wasn't possible, I spent some time googling and looking around for the setting.
After some digging, I found it in a hidden preferences file called .GlobalPreferences.plist in my user's Library/Preferences folder. I found the same file in the top-level /Library/Preferences folder, and then added the com.apple.mouse.tapBehavior setting as a class of Number with a value of 1 and saved the file.
Now my login screen works the way I expect it to, and as an added benefit, new user accounts now default to having tap-to-click enabled.
[robg adds: The easiest way to add this key to the global file is to have Apple's Xcode Developer Tools installed. Then in Terminal, you can cd /Library/Preferences and then type open .GlobalPreferences.plist, and the Property List Editor will open.]
Mac OS X Hints