Disable Dictation button on the keyboard of iPhone 4S / new iPad

OKAY, finally got it! The trick is to observe UITextInputMode change notifications, and then to gather the identifier of the changed mode (Code seems to avoid the direct use of Private API, though seems to require a little knowledge of private API in general), and when the mode changes to dictation, resignFirstResponder (which will cancel the voice dictation). YAY! Here is some code:

Somewhere in your app delegate (at least that’s where I put it)

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(inputModeDidChange:) name:@"UITextInputCurrentInputModeDidChangeNotification"
                                           object:nil];

And then you can

UIView *resignFirstResponder(UIView *theView)
{
    if([theView isFirstResponder])
    {
        [theView resignFirstResponder];
        return theView;
    }
    for(UIView *subview in theView.subviews)
    {
        UIView *result = resignFirstResponder(subview);
        if(result) return result;
    }
    return nil;
}

- (void)inputModeDidChange:(NSNotification *)notification
{        
    // Allows us to block dictation
    UITextInputMode *inputMode = [UITextInputMode currentInputMode];
    NSString *modeIdentifier = [inputMode respondsToSelector:@selector(identifier)] ? (NSString *)[inputMode performSelector:@selector(identifier)] : nil;

    if([modeIdentifier isEqualToString:@"dictation"])
    {
        [UIView setAnimationsEnabled:NO];
        UIView *resigned = resignFirstResponder(window);
        [resigned becomeFirstResponder];
        [UIView setAnimationsEnabled:YES];

        UIAlertView *denyAlert = [[[UIAlertView alloc] initWithTitle:@"Denied" message:nil delegate:nil cancelButtonTitle:@"Okay" otherButtonTitles:nil] autorelease];
        [denyAlert show];
    }
}