Here's a tip to help drill into a glazed tile without cracking the tile.

Start by using a nail to scratch away the glazing where the hole it to be drill. Scrape away the glazing until you get to the actual clay of the tile. Once you are down to clay, cover the area with masking tape. Then use a good high speed drill bit to drill the hole. Make sure you keep the drill at it's highest speed as you are drilling. The higher the drill speed the less chance you have of cracking the tile.