Right now getting Google voice requires an invitation. Getting an invitation means knowing someone who has invites and doesn't mind sharing one with you. The good news is that it may be easier for some to get invited if they happen to have a .edu email address.
According to Google, if you send them a request with your .edu address, they will shoot you an invite within 24-hours. Doesn't sound bad. What a charitable offer, Google helping students.
Here at iSmashPhone, we are huge fans of Google voice. While it doesn't have great iPhone integration, it works seamlessly on their Android phones.
If you happen to be a student and have such an email, you can drop by Google Voice for Students and sign up.