A couple weeks ago, the New York Times ran article about Heart Rate and Exercise Performance. (The second article by Gina Kolota that I've covered).
The article debates the known formula for figuring out your maximum heart rate - a number important if you wanted to tailor your workout to hit aerobic or anaerobic zones and improve your performance.
The article argues against calibrating your workout to these numbers, because the formula appears to be if not arbitrary, than at least inaccurate.
Personally, I wear a heart rate strap with my Garmin watch, and I rely on the numbers maybe half of the time. I find that when I run, my heart rate consistently hovers in the 85-90% range unless I really slow myself down. Funny thing is, I only feel like I am really pushing myself when I run my tempo runs. With biking, I live in the 60-75%, but because a lot of the effort is concentrated in my lower body (quads especially), I feel like I am working harder than when I run sometimes.
Either way, it's an interesting piece that reminds us to take all of the age-old wisdom of exercise and nutrition with a grain of salt.