doubleofive said:

It took you guys mentioning it for me to even remember that I had no idea what was going on because they hadn't mentioned it in the movies. How are they going to explain it now that they're done using it? It's not like they hadn't read the 7th book when they made 6....

Well, the mirror shard was introduced in Order of the Phoenix, the fifth book, and not mentioned in Half-Blood Prince, the sixth book.

When they were making the Order of the Phoenix movie, the seventh book was being written, so the producers of the film hadn't read the seventh book at that time. They thought the mirror was extraneous, so they cut it.

Here's what gets me - they were going to cut Kreacher from OotP, as well, but Rowling told them not to as he would be important in the seventh book. Why didn't she say the same thing about the mirror?!