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If it's in vibrate mode, it won't make the shutter sound.

I don't know of any other legit way.
 
If you're jailbroken, there is a soundfile in the directory uisounds, called photoshutter.caf. I have not investigated but I guess it's an aif format.

If you replace that file with a silent file, that would do it.
 
I pulled an app called "no shutter sound" from Cydia. I activate in Winterboard. Now I never have the sound.
 
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