You may be running out of disk space fast if you have small SSD start partition on OS X Lion if you leave MobileBackups enabled.
If you open Terminal and run the command "df -h" you might see the (otherwise hidden) directory .MobileBackups using up a lot of space.

This feature is described in the Ars Technica OS X Lion review for example. It is there so your laptop will run backups even when you're on the move with no backup disk attached.

To disable it, run the command "tmutil disablelocal" in Terminal.
To enable it, use "sudo tmutil enablelocal"

This MacBook (black 2 GHZ Intel Core Duo) would not accept the firmware password I just set via the Firmware Password Utility from the Recovery Partition in OS X Lion 10.7.2.

The following steps let me turn off the firmware password protection. It only works if you haven't forgotten the password you had set. I my case this means I had used the correct password but it wasn't accepted. Seems like a bug, maybe specific to my particular (and rather old) hardware.

1. Open Terminal and type diskutil list
2. Mount the Apple_Boot Recovery HD partition with diskutil mount disk0s3 (use the identifier you see listed)
3. In Terminal type: open /Volumes/Recovery\ HD/com.apple.recovery.boot/BaseSystem.dmg
4. In Terminal type: open /Volumes/Mac\ OS\ X\ Base\ System/Applications/Utilities/Firmware\ Password\ Utility.app
5. Now you can turn off firmware password - again, provided you know the password you had set