Here is another thought. Could you use the sound the fingers make as they tap on the table to figure out what key was pressed? This might be hard on a normal table. So a workaround could be a simple keyboard hardware device that emits a different inaudible frequency for each key. Now if two people are using this device near each other that wouldn't work. Wait. What about just using a keyboard with bluetooth. The iPhone can pick that up. That should solve all the issues. I am sure someone could make some money on a device like this.