The trick is to

1. Start with the ps3 in 'off' mode, when it is showing the 'red' light

2. Hold down the power button [ you will hear a first beep ], keep holding it for a few more seconds [ you will hear another beep ]

3. Now your display settings are reset. If you turn on the ps3 now, it will detect the new display source and configure it.

Here is a very detailed description of this

The trick is to

1. Start with the ps3 in 'off' mode, when it is showing the 'red' light

2. Hold down the power button [ you will hear a first beep ], keep holding it for a few more seconds [ you will hear another beep ]

3. Now your display settings are reset. If you turn on the ps3 now, it will detect the new display source and configure it.

Here is a very detailed description of this

You can also use this trick to reset the HDMI connection if you move the PS3 from a 1080p tv to one that cannot display 1080p.