This is one of the most mysterious task to be done on the phone. The manual does not document it, and only a handful of the users reported success.

After torturing my omnia, finally I can confirm the following is the valid methods to do a hard reset using the hardware keys.

When Omnia powered off,
hold onto the Call and Cancel buttons, press and hold the power button for a approximately 5 seconds until the reset screen shows up. If you do it wrongly, you will hear the samsung jingle and the startup animation.

When it is on,
hold onto the Call and Cancel buttons, poke the soft reset hole and the reset screen shall show up.

This is a snapshot of the reset screen.

By the way, this hard reset does not erase the 16GB memory. To reset everything, you could use the software hard reset by going to Settings > Hard Reset. There is also option to format the storage card.

One point to note though, if you decided not to format and press cancel, that will soft reset the phone.

P.S. Hard Reset means restoring it to the factory state, so all your files and programs will be removed! Backup your files before proceeding with the reset!