//Add two UIBarButtonSystemItemFlexibleSpace items to your toolbar, to the left and right of your items
UIBarButtonItem *flexibleSpaceLeft = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:nil action:nil];
UIBarButtonItem *flexibleSpaceRight = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:nil action:nil];
[toolbar setItems:[NSArray arrayWithObjects:flexibleSpaceLeft, settingsButton,deleteButton,aboutButton, flexibleSpaceRight, nil]];
[flexibleSpaceLeft release];
[flexibleSpaceRight release];
//Adding these as you would any other toolbar items will distribute space evenly between the two of them.