Does that guard against a double spend with them or a double spend on the blockchain as a whole?

i.e. could I (theoretically, if this got a lot worse) spend BTC at Satoshidice, count on them not being validated very fast because Satoshidice transactions are ignored by a proportion of miners, and spend the same BTC somewhere else?

I'm just an interested observer at this point, I have no BTC and I'm not into online gambling.

Currently, rule-of-thumb is to wait until a transaction has been verified by 5 miners. But instead of counting, most people just wait an hour. Some merchants are more thorough than others, so if you find one that doesn't wait for a transaction to be verified properly, you could certainly defraud them within an increasingly-large time window.

However, eventually the network will include one of your transactions in the longest chain, and the other will be discarded. So the double-spend will not be permanent.

> eventually the network will include one of your transactions in the longest chain, and the other will be discarded. So the double-spend will not be permanent.

The longest chain? Why would length be the determining factor?

If someone double spends a bitcoin, it could saved by one recipient and quickly be spent many times by another. The most fair way is probably to take the first spend.

Regardless, anyone ignoring a block of a transactions seems unsustainable long term if bitcoin is going to have any credibility. Seems like those with an interest in bitcoin should avoid even considering this.

Since it's possible (and increasingly likely as more miners join) that two different miners will add different transactions to their chains, the clients are all programmed to ignore all but the longest block chain. This is important because it's harder to spoof a longer block chain than a shorter one, so longer block chains are more secure.

How is it hard to spoof a longer block chain? If you intend to do so, it's trivial. You can create an unlimited length block chain and screw the short one that is perhaps the honest one.

This is one of the more disturbing defaults I've heard of with bitcoin.