1.3.0 added some security to the admin area through the use of a “secret key” to the url. Are you not able to use your admin, or are you just asking about the key? You can turn the key off in the System->Configuration->Admin panel, in the Security panel where it says “Add Secret key to Urls” yes/no drop down.
About the only thing forthcoming from magento team about this added feature is in the release notes:
“CSRF Attack Prevention (added form_key param to all admin Urls)”
addit- my guess is that the form_key is meant to help prevent outside scripting on admin functions by generating keys for a valid session and preventing admin settings to be changed without the valid keys.