Pretty much all of my cardio is done to the tune of my HRM. I found a pretty righteous plan in an issue of M&F several months ago, and really like it for spinning, elliptical, etc. However, once a week or so, I try to get out and hit the single track (mountain biking) for a change of pace. This means that I can't really stick to a specific HR plan, which is fine, except that sometimes I get pretty close to my max HR.
One of the trails I go on has some pretty beefy hills, and when I check my HR after getting over them, I can be in the 90-95% range (which is kind of scary since I was biking those same hills fatter and sans-HRM a couple of years ago!).
My question is - just what exactly happens when you hit your max HR? I'd hate to have to stop biking outside, or I could go to some wimpier hills maybe, but dammit it feels good to beat these hills that were giving me so much trouble years ago. ;-)