I think you need to use image. and then set the image be the buttons backgroundImage. such as :
navigationBar = [[UINavigationBar alloc] initWithFrame:CGRectMake(0, 0, 320, 44)];
UINavigationItem *navigationItem = [[[UINavigationItem alloc] initWithTitle:@"Detail"] autorelease];
UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 70, 30)];
[button setImage:[UIImage imageNamed:@"plain.png"] forState:UIControlStateNormal];
[button addTarget:self action:@selector(buttonClicked:)
forControlEvents:UIControlEventTouchUpInside];
UIBarButtonItem *buttonItem = [[UIBarButtonItem alloc]
initWithCustomView:button];
navigationItem.leftBarButtonItem = buttonItem;
[buttonItem release];
[button release];
[navigationBar pushNavigationItem:navigationItem animated:NO];
[self.view addSubview:navigationBar];