Friday night, my Co-Motion toppled over as I was about to put it on a repair stand and give the drivetrain a quick clean-and-lube before Battenkill. The fall - minor, slow-motion, usually harmless - bent the derailleur hanger toward the cassette.
Fortunately, one of my teammates is well-accustomed to bending derailleur hangers back into position. By leaving the wheel and derailleur on the bike, you can stick an allen key into the derailleur mounting bolt and use the leverage of the allen key to bend the hanger back into position. When the wheel is left on you can eyeball the alignment of the hanger, looking to see if it is parallel to the smallest cog on your cassette. Before and after doing this, tighten the bolts holding the derailleur hanger on to the dropout.
This probably weakens the hanger a little bit, so when I got back from Battenkill I went to DerailleurHanger.com and placed an order for a #58. You can search by manufacturer and model of bike, but it's not entirely complete, so you can also take a look at a very large batch of replaceable derailleur hangers and see which one matches the one on your bike.
Coincidentally, there was a short message on the team website encouraging riders to always have an extra hanger lying around.
My last experience with hangers was with an integrated (non-replaceable) hanger on my Tough Little Bianchi. While switching derailleurs late last spring, the hanger threads stripped (I use the passive voice because I am hesistant to say that I stripped the threads). I took it to a bike shop that had Helicoil inserts - they drilled out the hanger, inserted the Helicoil, and it was good as new. Helicoils are apparently common repairs in Big Serious Machines (like internal-combustion engines), so there's little doubt indeed that it would hold up to the stresses of a bicycle derailleur.
The rear derailleur, and its hanger, are fragile parts precariously hanging off of your frame. It would be a bummer to consider a race day, a long ride, or even your whole frame ruined because of a mishap, so know how to repair your hanger when it's busted and know what you can do when it's beyond your ability so that you don't prematurely consider your frame to be toast.