There is an option with AT&T to get a dumb phone data plan which is designed for phones with basic email/internet functionality. The data plan is unlimited however and called medianet and costs from only $10 - $15 per month depending on if you combine a texting (sms) plan.
The hack works by first signing up for medianet with a dumb phone. You may need to borrow a dumb phone if you don’t have one or buy a cheap phone on ebay for ~$20. The second part of the hack requires you to swap the SIM card you used in step 1 to a phone which is not listed in the AT&T database as a smart phone . A list of these can be found in numerous forums e.g. XDA. The current smart phone of choice is the unlocked Google Nexus (purchased direct from Google) – Congrats! – you now have an unlimited data plan for $10 per month. – Enjoy the savings!