and uh, maybe one of these (bad assss!)
http://www.amazon.com/exec/obidos/tg/detail/-/B000065CE5/qid=1029894956/sr=8-4/ref=sr_8_4/002-2876934-7640034?s=electronics

edit: while still badass, it's interesting to note that no one has yet built a heart rate monitor solely as a wrist watch... given that the wrist-strap , by it's very name, fits over your wrist, this is a little surprising... hmph...