EDIT: inefficient -> efficient, whoops

This is interesting, and gets more interesting if there is an efficient rental market for cloud hashing.

The attacker doesn't need to actually buy the hardware, they just need to rent it for a few hours or a day or whatever - just long enough to launch a 51% attack and steal money from a major exchange.

1. Rent enough hardware to create a temporary 51% pool.
2. start mining a private fork that gets ahead
3. Send in $X worth of currency A to the exchange (darkcoin say)
4. sell all of the A-coin for $X worth of B-coin (BTC)
5. withdraw all of the B-coin and wait for that to clear on the B-network
6. reveal your private fork to roll back the transactions and get all of your A-coin back

The attacker nets: $X + mining_revenue - mining_rental

In an efficient market, the mining rental cost becomes the same as mining revenue, so this scheme almost doubles the attacker's money.

The somewhat good news for bitcoin is that this attack is still easier for smaller market-cap coins, which would presumably start suffering from it first.