How to make return key on iPhone make keyboard disappear?

First you need to conform to the UITextFieldDelegate Protocol in your View/ViewController's header file like this:

@interface YourViewController : UIViewController <UITextFieldDelegate>

Then in your .m file you need to implement the following UITextFieldDelegate protocol method:

- (BOOL)textFieldShouldReturn:(UITextField *)textField
{
    [textField resignFirstResponder];

    return YES;
}

[textField resignFirstResponder]; makes sure the keyboard is dismissed.

Make sure you're setting your view/viewcontroller to be the UITextField's delegate after you init the textfield in the .m:

yourTextField = [[UITextField alloc] initWithFrame:yourFrame];
//....
//....
//Setting the textField's properties
//....    
//The next line is important!!
yourTextField.delegate = self; //self references the viewcontroller or view your textField is on

Implement the UITextFieldDelegate method like this:

- (BOOL)textFieldShouldReturn:(UITextField *)aTextField
{
    [aTextField resignFirstResponder];
    return YES;
}