This will probably work on most other HTC devices (daimond, etc).
The PTT button is I think one of the stupidest creations in Windows CE world. It allows one to program a list of people who - at an extra cost - one would be able to phone with a single button.
Without passing the judgement on the usefulness of this service, the chief annoyance of it is that it has a specially dedicated button which is often very easy to push accidentally (on the Cingular 85xx device, for example, it is right next to the power button), and - the worst part - is not remappable. Unlike all other feature-related buttons.
This last one I completely fail to understand, and I really, really, really hope that we (Microsoft) didn't make it so. Because if we did, it would probably be - for me at least - the one of the most embarrassing features to ever come out of the Redmond campus, right there with Microsoft Bob.
Anyway, this is what one needs to do make the PTT button do something useful.
(1) Disable PTT service. To do this, modify device registry value "Dll" in HKLM\Services\PTT by appending ".sav" to the value of the registry ("PTTService.dll.sav"). This will prevent the PTT service from getting loaded - and will make it easy to restore should you ever want it back (just remove the ".sav").
If you have Visual Studio, you can use Remote Registry Editor in Visual Studio Remote Tools to edit the registry. Otherwise, google for "Windows CE Registry Editor". There are plenty of tools in existence.
(2) Open File Explorer on the device. Go to \Windows directory. Click-and-hold on voicecommand (you will have to scroll down a long, long list of files). Select "Copy" from the context menu. Then scroll down to the very bottom of the screen, and click-and-hold on the empty space. Select "Paste" from the context menu. Rename the resultant shortcut to "Short_PTT".
Note: this works only because voicecommand is already a shortcut. If you want to map an application (executable file) to the button, the instructions are as follows. Click-and-hold on the name of the application. Select "Copy". Then scroll down to the very bottom of the screen, and click-and-hold on the empty space. Select "Paste shortcut" from the context menu. Rename the resultant shortcut to "Short_PTT".
(3) Reboot the device to eradicate the PTT service. You are done.