How does it work? You create a coinbase account, you then link this unverified account with your bank account using these 2 options: 1.you can hand over your online banking u/p for instant verification or 2. you give them your account # + routing number for them to make 2 small deposits to verify that it is in fact your account. You can see it here: http://blog.coinbase.com/post/34357253898/you-can-now-buy-and-sell-bitcoin-by-connecting-any-u-s

So far so good right? Wrong. Exchanges can't use ACH its plain and simple - anyone can do a chargeback at anytime.

Now lets just imagine that they make $200k or so in transaction fees a month which i estimate they should have hit by now, i'm going to show you how one scammer can take that all away in a bit instant... (pun intended)

6 steps: 1. Open a shell company 2. Open a bank account and deposit 25k in seed money 3. Open a coinbase account transfer your money in and buy some coins 4. send your coins out to MtGox or another exchange and cash out 5. Deposit the money back in your account and repeat second part of step 3 + 4 6. Walk over to your bank's local branch and file for a chargeback claiming you dont recognize the transaction.

Now you're left with both your cash and your coins, which technically are liquid enough (unless MtGox goes rogue but then everyone is fuck'd as well..)

Coinbase, i wish you gents luck and i think you should not rely on ACH.