If it's just through Wifi, you can get Fongo for free. It works decently enough, though they charge for SMS. But for that you can get Pinger(aka Text for Free) and send texts. They charge for voice calls, though incoming is free but the quality isn't as good as Fongo. So i use both, the only issue is you have to give people 2 numbers, but I don't really give anyone my Fongo # as that kinda defeats the purpose of staying of the grid
. Good for making calls when you really need to though.
EDIT: and now I read the article... So they run through the cell network if you are not on WiFIi. The price looks OK, though a 100mb/$9.99 price would be sweet. I might even consider a sim card then...