The regular client doesn't perform a check of the live transaction queue, but it could be done. I'd be willing to bet that it will be a standard check on POS systems long before Wal-Mart starts accepting Bitcoin at their registers. And a slick POS client could also send the transaction to all of it's peers except one, and if that peer offers back that same transaction then it's safe to assume that the transaction is good. Even if there is a double spend attempt underway, if your own POS system has a decent spread of peers and your last peer then sends you the correct transaction, odds are high enough that your transaction was first (and therefore most likely to come away with the actual bitcoins) and therefore acceptable. This would be acceptable risks for sales values under $50, and is similar to how credit card companies handle charges under $50.
Such a check wouldn't take nearly 15 seconds under normal network loading conditions, but a 15 second timeout would be reasonable.
That is an interesting idea. I'm not so sure that gives you that good of certainty. If there was a malicious user, it would be their goal to send out the tx they dont want to see accepted as much as possible through the network. Although it would provide protection, some users would still exploit the system. Whether that is enough for places to take the losses, I don't know.
No, not certainty, but high probability. If the POS owner chose his peers well, it would be a quantifiable probability; but it would be pretty certain under random peer selection as well. High enough that I'd be willing to offer vendors insurance against a double spend fraud, and I'd be running the POS super-node. I can forsee an niche industry of Bitcoin processing companies offering such insurance to small vendors, but chains such as Walmart or Sears are going to do such thinkgs for themselves.
In any case I don't see a future where every normal user is using standard bitcoin nodes. The network probably won't handle that and I doubt merchants will want to run their own bitcoin instances all over the place. Much more likely is a set of payment processors using bitcoin as the currency of exchange.
That's it exactly. In the short term, Bitcoin vendors are likely to have normal clients running in the store, but ultimately such functions are likely to be consolidated into datacenters with special POS light clients in the store. This is pretty much how credit card transactions are handled these days. The important part is that an independent vendor always has the realistic option of setting up a local full client if his service company ever gets uppity, and the customer also always has the option of setting up his own full node if he ever has reason to distrust his online wallet bank. Keeps these groups honest.