Is Login Password secure?

anyone with UNIX knowledge can startup your Mac in Single User Mode and delete /var/db/.AppleSetupDone to restart the first boot Setup and create a new Admin account and then look through or delete your account.

someone can also startup from the install disc if you or they have one handy and simply erase and install or do an archive and install and make a new admin account and then look through your account.

there is a very slim chance that this will happen though. you can add a Firmware password to your Mac to be safe by booting into the install CD and choosing Firmware Utility from the Utilities menu. this will stop any other boot methods other than normal booting without a password.

 

The login password can be cleared and reset with very little difficulty if one knows what they're doing. If you enable FileVault it will dramatically increase the difficulty because not only does it encrypt your user account, the password can only be reset if you know the master password that is set. The trade-off is that if YOU ever forget it...well, just don't forget it...

 

A strong Firmware password is better.

http://support.apple.com/kb/HT1352

 

Anyone with physical access to the computer and has a little knowledge can access the information on the Hard Drive - with the possible exception of File Vault data. Your best bet would be to make the computer as physically secure as possible - i.e. locked in a closet or drawer with strong locks.
Login passwords and Firmware passwords are very easy to circumvent with unlimited physical access to the device.