If you have an HP Elitebook 8440p (and specifically, model WH256UT#ABA, but possibly others), there is a bug in the Windows XP 32-bit nVidia driver that causes the screen to appear to lock up when it blanks.
To test this, set the screen to blank after 1 minute. Your screen will go blank, but will not come back to life.
If you plug in an external monitor, voila! it does come back to life.
The problem is the video driver.
Either use an old video driver, or use one that HP hasn't released yet, version 261.55 (shows up in device manager as 22.214.171.12455, dated 3/9/2011).
Here is the driver: 261.55_H12298_XP32.zip
Note: HP's support site should have the latest, but for now, I have the latest.