How can I switch off "hold home button" voice control?
In Settings &gt; General &gt; Passcode Lock (Yeah, a weird place, but hey ho) you can limit the voice control to music options only, and turn off voice dialling.  This doesn't disable the long press option, but it does mean that at worst it can annoy you rather than embarrass you...





Note that for Jailbroken devices, there seem to be a number of tweaks and hacks that will disable the function entirely.  The following links are from a brief google, and I haven't tested them as I do not jailbreak but they look promising:


http://modmyi.com/cydia/package.php?id=28396
http://www.youtube.com/watch?v=JzrJGhGbr2s