If you haven't been following along, here's why this matters:

Tethers is a sole-source cryptocurrency, pegged to the US dollar. Bitfinex produces it, though they're cagey (some would say outright lying at times) about the level of involvement.

The primary purpose of tethers is money laundering, even more so than cryptocurrency generally. Bitfinex was cut off from the US financial system, which makes it impossible for them to clear USD-denominated wires. Their clients have ~$400M of USD on deposit with Bitfinex. Their solution: issue a cryptocurrency which is claimed to be 100% backed by USD and say that it is redeemable 1-to-1 for dollars... we just can't actually physically give you the dollars.

Incredibly, this has worked so far. Bitfinex has issued approximately $600 million in tethers, all but $10 million or so in the last 6 months. They're usable on a handful of exchanges, for the purpose of buying BTC and other cryptocurrencies.

Tether claims that someone has stolen 5% or so of their $600 million tethers -- the digital claims, not the underlying dollars sitting at their totally-exists-we-promise bank account. They've made a technological change to the Omni client to disallow transactions on the stolen tokens, but there is no guarantee that they succeed in convincing all parties to use this.

The nightmare scenario for them is 1+ exchanges say "Well, actually, we rely on your money actually being money to list it here, so pick: we delist you or we don't, but we don't have any incentive to apply that patch." [0] The thief immediately exfiltrates to Bitcoin, and suddenly 30 million hot tethers are contaminating the money stream at the exchange, and they cannot be conveniently disambiguated from clean tethers, because money is fungible.

Hilarity then ensues, for values of hilarity which probably mean "bank run" on a bank which is structurally incapable of paying out most holders of money.

[0] Why does Bitfinex care whether their cryptocurrency is listed at other exchanges? Because they need to launder money to support their exchange business. Tethers are institutionalizing a sort of crypto-hawala (or crypto-Liberty Reserve), allowing the physical transfer of real money to happen at legal remoteness to the cryptocurrency exchange.

Bitfinex has an order book filled with something people want. A way to get access to that orderbook is to say "Bitfinex, I want some tether, how do you sell them to me if you can't accept a wire?" Bitfinex might say "Are you a trustworthy US VC? Spiffy. Move $20 million from your left hand pocket to your right hand pocket. The right hand pocket is now ours; here's $20 to $21 million in Tether, which are good for BTC at your favorite exchanges. At some time in the future, a trustworthy US VC other than yourself is going to ask you to buy some tether from them at par value. You will do that, and pay them from your right pocket. If a regulator asks you about this transaction, you bat your eyes and say 'Oh, sophisticated investors doing a cryptocurrency transaction, nothing to see here.'"

Post-script: Is this good news for Bitcoin? Oh this is great news for Bitcoin. If you don't believe Bitfinex's $600 million in liabilities are worth a copper shilling, the only option for getting your value out of Bitfinex is to swap your liability for Bitcoin, which drives up the price of Bitcoin at the margin.

This is exactly what happened in the final months of Mt. Gox.