One of my favourite features on the HTC Touch Pro I am trialling is the screen that appears when I plug the device into a USB port, offering Activesync, USB Mass Storage Mode (which makes the microSD card available as a memory stick) and Internet Connection Sharing. The software is very convenient, and remembers you last choice, so if you do not chose immediately it will do the last chosen action.
Unfortunately HTC does not quite see the HTC Touch Diamond as the same caliber of business device as the HTC Touch Pro, and that device lacks the Internet Sharing option in the pop-up screen. The normal ICS app still works, but needs to be launched separately.
As usual, the good folks at XDA-Developers have helped out, and brought this great convenience also to the HTC Touch Diamond. By installing a cab, found in this XDA-Developers post, you too can have mindlessly easy internet connection sharing. At present the cab only works on English devices, but keep an eye on the thread for other language support.