-From the movie Ever After
The above bit of dialogue was actually based on fact-notably the line "some claim the shoe was made of fur. Others insist it was glass." In Perrault's version, he used the word "verre," which is "glass" in French. Somehow a rumor got started that this was a mistranslation of "vair," or "fur," and this was perpetuated for a while and even stated in the Encyclopaedia Britannica to be the case. Perrault officially wrote about a glass slipper, although it really doesn't matter in a way because his version was not intended to be based on oral tales, but his own literary story. It's likely that the subsequent versions of Cinderella including glass slippers were influenced by Perrault, since before then the shoes were any valuable material, as many fairy tale objects are (glass, gold, silver, diamond, crystal, etc.) The jump doesn't seem to be a hard one to make from one valuable material to another-I have a pair of gold ballet flats and multiple people have commented that they're like Cinderella's shoes, although they probably are not aware that many Cinderellas did indeed have gold shoes.
But as for glass itself-it can be seen as a symbol of virginity, as it can only be broken once. Jewish weddings feature the groom crushing a glass under his foot for good luck. With this in mind, a glass slipper as Cinderella's identifier makes perfect sense.
Sources: Paul Delarue-"From Perrault to Walt Disney: The Slipper of Cinderella" from Cinderella: A Casebook
Alan Dundes-his introduction to the above essay
*The slipper in the image is available from Disney for $300, if you have more money than you know what to do with...