I've had mixed results with enabling it for the login screen between my macs.
Here's how I have gotten it to work:
- Go to System Preferences (as an admin user) -> Users and Groups -> Login Options
- Click the lock and enter your admin credentials
- Next to Network Account Server click Edit (or add, what ever comes up if you don't have a server already set up) and then click "Open Directory Utility"
- Click the lock again and reenter your credentials
- In the menu bar, click edit and then click "Enable Root User"
- Set up a password for the user, and then log out of your current user
- At the login screen, you should have a "Other..." option next to your users. Type in "root" as your username and use the password you set up.
- Enable tap to click through System Preferences.
- Log out, and go back into your normal user, and open up Directory Utility again
- Click on the padlock to authenticate in Directory Utility, and then go to edit on the menu bar, and click "Disable Root User"
For security's sake, make sure you disable the root user!
Now I mentioned at the start I've had mixed results for this. It worked fine on my Mac Mini 2012, but it didn't seem to work on my MacBook Air 2011. The thing that I can think of primarily is that the MacBook Air's trackpad is built in and connected via USB, whereas the Mac Mini's trackpad is bluetooth.
Why does this matter, you ask? Did anyones else notice that the Terminal command that was used to change the system default to having Tap to Click on seems to store the setting in an entry that references AppleBluetoothMultitouch.trackpad?
If thats the reason, then that would be why some people report the terminal command works and others say it makes no difference.
I'm going to dig more in this, I wonder if there is a similar AppleUSBMultitouch.trackpad entry somewhere...
Just confirming, I connected my Bluetooth Trackpad to my MacBook Air (it is surprisingly difficult to remove a trackpad from bluetooth preferences on a desktop mac by the way) and I can Tap to Click on the login screen with the bluetooth trackpad, but not the built in one. Innerestin'!
Oh, for comparison to your systems, both my Macs are running 10.8.4, have no startup items at all (I like my speedy boot times) and have no third party Kexts