If you use Crank Brothers pedals, you should read the instructions. The pedals need to be rebuilt or serviced every 100 hours. This is an easy thing to do, much easier than recovering from a serious injury!
Update 14-SEP-2008:From the CrankBrothers instructions...
For maximum durability (and depending on your riding conditions), you should re-grease your pedals after every 100 to 300 hours of riding. Wet or muddy conditions are the most severe. Re-greasing only takes a few minutes per pedal and can be done with the pedal still on the crank arms. Use any high quality bicycle grease available at bicycle stores...If the pedal becomes loose or feels gritty when it turns, then the pedal needs to be rebuilt.
You also have to be careful when you purchase a rebuild kit, that you get the correct kit for your pedals. Crankbrothers makes a large number of different models that require different rebuild kits. I carry a rebuild kit in my bike tool kit. Regreasing pedals once a month sounds about right for my mileage. I use a rebuild kit every once and a while.