There is almost a religion built up around the idea of an entrepreneur's elevator speech. You're supposed to have it ready at all times, just like your business card. You should be able to rattle off your elevator speech in thirty seconds - some say 15 seconds. It's like your audio business card.
When I think about your elevator speech and its necessity for entrepreneurs, the first thing I think about is this. What question is your elevator speech the answer to? I see the elevator speech as the answer to only this question: "What's your elevator speech?"
The trouble with almost any other question - such as "What do you do in your business?" or "What sort of business are in you in?" is that these questions, and the conversations that contain them, are very situational. So for one situation, you might talk about one aspect of your business, and in another context, you might lean a different way.
Here's what I think: I think that you should know about your business. I doubt very much that you don't. You should listen carefully to any questions that are asked of you, and look for other cues - in a person's personality, what he or she seems to be interested in, and so on - that will help you answer a question like "What does your company do?" appropriately.
Canned speech is never the right answer. An audio business card is an aural insult. No one is starting a conversation with you in order to be the passive receptacle for your how-great-I-am speech. Listen, and respond. If your conversation partner wants to know more, he or she will ask. Let the conversation be your guide, and leave the elevator speech at home.