ABC News wrote a good piece on how to go about asking your employer if they will let you telecommute. They cover some really good points.
I think the most important thing mentioned is to make sure your performance is outstanding before you approach your employer about telecommuting. They need to be able to trust you and the integrity of your work ethic – and they also need to strongly value your work so that you can be certain they will consider your request favorably.
I also like that they ask you to honestly consider if you will be able to perform your job from home. Are you easily distracted? Do you have an office in your house or will you be stationed in the center of a chaotic household? You’ve got to prepare if you want to work from home. Make sure you approach the whole concept professionally. If you take professional steps before you approach your employer, they will be much more confident in making this leap with you.