Garthanos
Arcadian Knight
I've toyed with this in many different iterations. Here's something I wrote almost 2 years ago when I was working on it. Part 1 and 2 are background information that works out how and why the 4e systems works the way it does. Part 3 has the injury mechanics.
Part 1
Part 2
Part 3
Hope this inspires you to use it if you've been looking for a way to do injuries.
Hmmm the takes something significant when it goes below zero is very much a call back to 1e rules (in 1e at that point 1 week recovery was required even if your cleric fixed hit point loss.. ie you were still very much wounded even if you had full hit points).