To change the default user profile on a Mac with 10.5 or 10.6 …

First create a user account and setup all the preferences you want. Set the Dock, change the desktop background, set up application preferences like the homepage for the browser, etc. Then log off that user account and log into an admin account with root privilege. Open Terminal and run the following commands in order: (The first one will ask for the root password , usually the same as the admin pword…)

sudo rm -R /System/Library/User\ Template/English.lproj
sudo ditto -rsrcFork /Users/labuser /System/Library/User\ Template/English.lproj
sudo rm -rf /System/Library/User\ Template/English.lproj/Library/Keychains
sudo chown -R root:wheel /System/Library/User\ Template/English.lproj

The first command deletes the current, default user profile. The second copies the profile from the “labuser” account (change this if you use a different user account name). The third command removes the keychain file so that the new users will get a new keychain when they log in. The last command sets the file permissions.