If you’ve upgraded to OS X Lion be aware that it’s now easier for anyone with physical access to your system to break in. Researchers have found what seem to be accidental oversights in the protection of passwords in Lion, making it easier for them to be read and changed by unauthorized users.
To protect yourself make sure you have some standard security measures in place, including disabling auto login, enabling screensaver passwords, disabling guest accounts, and disabling admin privileges for regular users. Luckily these are all pretty easy to do with OS X. Directions are in the first link below.
http://www.defenceindepth.net/2011/09/cracking-os-x-lion-passwords.html