Since performing a hard reset is completely different on each device (and somewhat hard to find instructions on how to do), here are the steps you need to restore the factory defaults on each device.  Here is a fairly clear disclaimer from the AT&T Tilt's user manual that explains what a "hard reset" means:

Hard Reset (a.k.a. full reset): A hard reset should be performed only if a normal reset does not solve a system problem. After a hard reset, the device is restored to its default settings - the way it was when you first purchased it and turned it on. Any programs you installed, data you entered, and settings you customized on your device will be lost. Only Windows Mobile® software and other pre-installed programs will remain.

AT&T Tilt (a.k.a. HTC 8925)

1. Press and hold the left SOFT KEY and the right SOFT KEY, and at the same time, use the stylus to press the RESET button at the bottom of your device.
2. Release the stylus, but continue pressing the two SOFT KEYs until you see a message on the screen saying "This operation will delete
   all your personal data, and reset all settings to the manufacturer default settings. Press SEND to restore manufacturer defaults, or press any other button to cancel.
3. Release the two SOFT KEYs, and then press the button on your device.

Alternatively you can go to Start > Settings > System > Clear Storage, and it will do the same thing.

iPhone 3G

1. Start with the device on
2. Go to SETTINGS > GENERAL > RESET > ERASE ALL CONTENTS AND SETTINGS

NOTE: This one takes abnormally long to reset everything back to the factory defaults (around an hour).

Palm Treo Pro

1. Start with the device on
2. Take off the back cover
3. While holding down the END button use to stylus to tap the RESET button (located on on back in the bottom left)
4. Keep holding down the END button until you see the "Erase all data?" prompt.
5. Press Up.

HP iPAQ 910

1. Start with the device off
2. While holding down the OK button (located on the right side) and the VOICE COMMANDER button (located on the left side) use the stylus to press the RESET button (located on top)
3. Release the OK and VOICE COMMANDER buttons when "Clean Boot" is displayed on the screen.

Motorola Q 9H Global

1. Start with the device off
2. Hold down the * and E keys for 5 seconds while turning on the device