More than likely they are using PDAnet app. Which enables you to tether you smartphone to the laptop. The app is available for Android, iOS (iPhone), or Blackberry. Does require root access for the iPhone, but not for Androids. Not sure about Blackberry.
I spoke to a representative today and she told me that as long as you have an unlimited plan, it won't cost anything until the first quarter of 2012, and then it will be an extra charge... $20/gb like ptribb mentioned. But as of right now, it's free (as long as you have the unlimited plan).