Would bullet proof glass help deter burglars? How does it compare to normal glass?
There is a product called Window Security Film which is a thin (a couple mm) film that adheres to the window and makes it exceptionally difficult for someone to break.  Many lock smiths/security companies can install it.   Here's a video of it in action: http://www.youtube.com/watch?v=vYdVK3BqPfk

One of the biggest "problems" with it however is that because it's invisible, people will still try to break the glass.  For this reason, often it is installed along with window bars since this provides a visual deterent, but they are pretty ugly for residential installs.