Bicycle Mechanics - Crack in Rear Derailleur

After cleaning my drive train and hanging up my bike I noted this crack in my rear derailleur. Is it something I should worry about? I'm not sure what the purpose of that wing part is, but it doesn't seem to be carrying any loads.

The bike (Trek 520) is less than a year old. I'm guessing the crack is the result of a manufacturing defect. Is getting it replaced under warranty typically much of a hassle?

http://i969.photobucket.com/albums/ae176/gECHO/IMG_0956b.jpg

I suspect that it's a stop to keep the RD from swinging back too far when you remove the wheel. The front side will engage the stop tab when you pull the RD back (or when the upper body spring on some models does so). It's probably cracking from a bit of hard banging into the stop tab, but it's hard to tell from the photo.

If it's cracked, it could fail, replace.

For warranty, can't work out how you did it, you would have to ask your dealer

I dropped it off at the shop tonight, the head mechanic was intrigued by the crack. He needs to show it to someone else tomorrow to see if it might be covered under warranty. I don't need that thing flying off at 40km/h.

On my first mountain bike I had the stem fail once due to metal fatigue and the handlebars came off in my hand while riding. I couldn't steer but still had the brake levers, which I applied very gently.

Yup as FB mentioned - its a safety feature that puts a bit of extra play in the system and might give you a chance to back off the pedals if your chain`s been derailed.

The upper casting in the Shimano Tourney models is plastic and its not unusual to see that stop completely sheared off due to a drive system mishap that scrapped the derailleur.