Can your rims wear down from too much braking? I have a question about rims based on information in another question.  The person asking states that he doesn't want his rims wearing down.  Is this really a problem?  Assuming you don't wear the brake pads down to the metal, can the rubber pads actually wear down rims?  Does it make a difference if you use aluminium, steel, or carbon rims?  My rims are aluminium with  tiny grooves on them.  Is there anything I should look for to ensure that they aren't worn down?  I have over 10,000 KM of mostly city riding on my bike.
Short Answer: Yes

Longer Answer: Yes, but maybe not for the reasons you are thinking of. Rim wear is mainly caused by dirt and grime that gets caught between the brakes and the rim, rubbing it like sand paper. The most important thing to do is clean your rims and pads as often as possible. For that matter clean your drive train as well.

As far as different materials go, it is a lot easier to bend back steel than aluminium or carbon. So if you do have a steel rim you are better off if something goes wrong. There is a tool called a dental caliper (pictured below) that can help you find out if your rim is worn down.

Here is an interesting story and some good info on bent rims.