Quote from: casascius on February 14, 2011, 08:39:12 PM

The only way grocery stores could get on the network would be to have some sort of "MYBITCOIN"-like intermediary that does all the transaction processing, most of which would have to happen off the block chain.

Right... so then the question is "will the MYBITCOIN-like intermediary be able to verify transactions quickly without opening themselves up to systematic fraud."

Without requiring users to pre-deposit funds with them, because users ain't gonna do that.

Here's another possible simple rule for miners that might work (but, as Hal said, requires Deep Thought):

"When I see a new block with transactions that I didn't see broadcast previously, mark those transactions as suspicious. If I see double-spends of those transactions, stop building on that block-- assume it is cheating. Switch to the previous block (or alternate block if there's a block race going on)."

Miners won't try to rip off a grocery store for $50 worth of groceries if doing so makes their $50+ bitcoin reward for finding a block disappear.

This rule would also give miners a strong incentive to detect and refuse to include EITHER side of a double-spend in their blocks (if they get both spends while they're working on the block).