We implemented this to be primarily used in Workflows, along with the ability to Tweet, to give you more flexibility in how you communicate with your clients. Currently we can't use your mobile number for SMS but we are exploring technologies to enable you to, so that will come. In the interim you'll find that it is still extremely cool to use in Workflows. Just alert the recipient the first time you use it, that it is indeed from you and how you will be using it.
When you choose a number, the goal is to get your area code. In some cases you may be in an area code that is so popular that there are no numbers left for yours. In that case, please pick one that is closest to you.
Note that you have an option in Settings, SMS Services to enable you to be notified by email or text that you have received a text.