To cheat you, when he generates a block, he doesn't broadcast it. Instead, he runs down to your store...
I think the real danger is that a large mining operator would create a side business selling space in their blocks for these types of intentional double-spends. When they generate a block they could send a text message to a bunch of people saying "try to spend NOW".
I wonder if there's some way to discourage that kind of anti-social behavior; could the network detect that was being done and "shun" that miner's blocks?
Well, it's detectable, no? The ripped-off merchants will complain and we will likely find out (at least if the operation is continued), which pool is the bad guy. (I'm assuming it will be a pool operator doing this side-business). People would leave the pool and therefore harm it, likely more substantially than can be made up through the side business. Therfore pool ops are unlikely to pull this off.
In case of a large solo-miner it's not so simple, because identifying the block source would be a lot harder, especially if he takes precautions (no?) and also because he doesn't suffer any harm from "people leaving the pool". In this case the only option might be to "shun" this miners blocks. Not so easy. The blocks might be detected by using a rogue customer using the "service" and looking for the specific transaction. Unfortunately, it would be easy to identify the rogue customer, because we'd need a way to broadcast this information and the bad guy could listen to that as well and simply block our rogue customer, right? Also there might be possibilities to abuse the system to harm competing miners.
While I can't think of a straightforward way to protect against this kind of anti-social behaviour, I also think this kind of behaviour is currenlty very unlikely to occur. This might change in the future, though.
