[lloeki]

> "if you're sure your hardware hasn't been tampered with, this should physically disconnect the microphone."

Why so? Most soundcards merely detect an impedance change and flick a bit to switch from internal microphone to line-in but you can have full control of that bit by software. That's actually what the prefpane does: even with something plugged in the line in, one can switch back to the internal microphone as a source. (EDIT: apparently it goes hidden on new hardware revisions but the functionality is there, just boot into Linux and do some poking on the soundcard to see what's really available)

[calloc]

You can still insert a barrel jack, this time with 4 rings instead of 3 at which point the microphone is disabled.

For example, Apple headphones for your iPhone can be plugged in and used for that purpose...

[pavel_lishin]

Only if you cut the microphone off, of course. :)

[stcredzero]

You can buy a 4 ring 1/8" plug adaptor for headphones. Some of these adaptors come with no mic.