Bitcoin blockchain issue - Mt Gox Bitcoin deposits temporarily suspended
Ok, here's what happened and what it means. Bitcoin is built on a chain of blocks, each of which contains a set of transactions, the hash of the previous block, and a cryptographic proof-of-work which takes a lot of computing power to construct. Whichever block-chain is highest (that is, has the most total computing power invested in it) is considered valid, and has more blocks added to it by miners. People "mine" by taking outstanding transactions, writing them into a block together with the hash of the current highest-numbered block, performing cryptographic proof-of-work and publishing their new block.A block was mined (by the Slush mining pool) which is accepted as valid by 0.8 clients, but rejected by 0.7 clients. More than half of the mining power was on 0.8, so the longest chain includes this block - but the 0.7 clients reject it, and have built a side-chain.  The developers have asked all miners and mining pools to switch from 0.8 to 0.7, so that the 0.7 chain will grow to be longer than the 0.8 chain; once this happens, all Bitcoin clients will agree that it is the real one. In the meantime, however, merchants running Bitcoin 0.8 may be targeted by double-spend attacks: if they receive money in a transaction that exists only in the 0.8 blockchain, the same money is spent in a different way on the 0.7 blockchain so the transaction can't just be copied over, and they act on the receipt of money by sending goods before the situation is resolved, then they could lose money. This is why MtGox has suspended Bitcoin deposits. (This can only happen if the coins were sent by someone who is using malicious, nonstandard software; transactions made by honest users will be copied to the 0.7 fork and mined without any issue.)The height of the two chains can be monitored at https://coinbase.com/network/blocks . Currently the 0.7 chain is 13 blocks behind, which (since the normal mining rate is 6 blocks per hour) sets a lower bound of about two hours on resolution - assuming everyone switches their mining power immediately - but realistically it will probably be more like 6-12 hours.