I'd like to point out, that I made an effort to make it fairly straightforward to run this application. Granted, it's only tested on Linux for the moment, but it has very few dependencies and can run in combination with a standard Bitcoin client (so no patches needed or anything). You can use any Bitcoin mobile client, unless you want to test the green address feature. Then you will need BitPay with my patch applied (see the GitHub link for more info about that). But hopefully, the version of BitPay in the Android market will soon be updated to make this step easier as well.

What do you think, how cheap could a hardware solution based on this be put together? I'm thinking old, used laptop (maybe an Eee PC) plus smallish external monitor. Maybe all in all for $150? Maybe less? .. I think we could reach the point, where this is a straightforward and fairly inexpensive way to get a brick and mortar merchant set up for Bitcoin payments. I think maybe the only missing piece in the puzzle would be automatic exchange to the merchant's local currency (e.g. USD) to have it be a complete worry-free (regarding exchange rates) solution. It might be interesting to interface with a Bitcoin exchange here.