Speakout Wireless no catches, unlimited but slow 3G data for $10/month, no voice plan required is my conclusion after a month. Slow means under a 1Megabit sometimes  64 kilobits! But more than good enough to run GPS and do maps and geotag photos. Recommended if you are cheap :-) and want pay as you go data! Or just want a second phone with data and don't require voice! Only tested with a Nokia N8 with Nokia Maps and Pixelpipe but other folks have got it working on Android and iPhone. (see my previous speakout post for setup details).