The short of it solution... from a Terminal window as a user with Administrator privileges type:
sudo tmutil disablelocal
Enter your password and it will stop local backups AND delete the backups currently on your internal disk.
There was an option in the Developer release to turn this feature on and off via the System Preferences Panel for Time Machine. It was removed from the final release. It was also originally also suppose to store internal backups and then transfer those backups to the external TM disk when a laptop was back in range or reconnected to the external disk. These features were changed in the final release. It does not, nor is it suppose to (at the current time) transfer internal backups to your external TM disk when you are in range or reconnected. It's not an option at this time and the only way to get the space back is to wait for TM to remove them, or remove them manually. It is designed to retain the internal backups and use an algorithm to manage how long to keep them now.
Running the above listed command will disable internal backups, yet still allow Time Machine to function as it did in 10.6.8 ie: you don't have to turn TM off completely in System Preferences. Look at the System Preferences window for Time Machine and you will see a description (in the lower right) of what it does, and mention of attempting to keep local backups. Once you run the above command, that text will change and no longer mention the local backups. Hope this helps.