iPhone app editing mode

I need some help with my application.
First I have a TableView which pushed on another View. In the second View I have another TableView and here i have to be able to insert rows.

What I want to to is to replace the back button with an add button when editing mode is on.

How do I do that?

I've tried this in viewDidLoad:

Code:

if (self.tableView.editing) {
		NSLog(@"editing");
		
		UIBarButtonItem *addButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAdd 
																				   target:self
																				   action:@selector(insertNewObject:)];
		self.navigationItem.leftBarButtonItem = addButton;
		[addButton release];
	}