Heart Rate ear phones. I'd be disappointed if Apple did come out with such a product, but looks like others are.
Sounds like a gimmick - you have to be wearing your ear phones in order to get heart rate stats. Assumes you always want to wear ear phones when doing an activity - and which limits the target. Sure, in a gym or running, may be fine, but other outdoor activities such as - skiing, cycling, types of boating, you probably wouldn't to - when you need to be aware of your surroundings.
Having a heart rate strap, or band is much more practical.