Impossible has been done again by the folks at XDA-devs. They have been able to add/activate 850mhz band to the HTC Touch Diamond. All you need to get the Quad Band radio available there.
Please note that the faint-hearted should not do this. If anything goes wrong, you may be left with a bricked diamond.
..and I apologize not to mentioning the URL of ROM as it is against our website’s policies. But, hey it is easy :!: