I've been wondering about this, too. On occasion I would like my kid to have a cellphone, but don't want to get him (and he doesn't need) a full-time line.
I thought Tracfone might work in this situation, but frankly haven't been able to figure out how it works, if it works, etc.
I have a Virgin Mobile phone that I got at Walmart...I pay $15 every 90 days or when my account drops down to five dollars. I rarely use my phone at all, but wanted it for occasional convenience and emergencies.
I ordered a phone off the virgin mobile website for $20 and it came with $20 worth of airtime, so I essentially got the phone for free. Free shipping too, if you order it from the website. I gave it to my husband. It's on the same plan as mine...after 90 days, it adds $15. The minutes roll over that you don't use.
http://www.virginmobileusa.com/