Ahh, there _are_ heart rate monitor accessories for the iPhone already
I was operating under the impression that nobody had created a heart rate monitor system for the iPhone yet. This seemed illogical to me as it is such an obvious thing to do.
As it turns out, there is one. I found it today at Smheartlink's site. It looks like a great product, but it is a lot of money to spend, especially after you have already purchased a HRM belt.
Its a bit disappointing to see this considering my app will do much the same stuff, but I still see a niche for my app, as it doesn't require you to charge and carry another device around with you, plus it will be a bit cheaper :)