Carriers like Virgin Mobile offer unlimited 3G data for $40/mo, month-to-month.
Clear offers unlimited 4G data for $45/mo, also month-to-month.
You can get unlimited 3G/4G for $55/mo with their spot+, also contract-free from Clear.
I stream over Pandora and Netflix regularly, and have to have unlimited data. I used to use AT&T's unlimited 3G for $30/mo, but the signal/service was too intermittent and slow for me to keep it. Both Virgin Mobile and Clear gave me stronger and faster service.
These are the only unlimited mobile data plans I know of in the US as of this posting. I plan on adding second unlimited data plan for my new iPad 2 when I get it.