[IanChiles]

The fun thing about Bitcoin is the immense latency in moving money (USD) between exchanges, which has at minimum 4 days of transit. This makes just about anything involving multiple exchanges or high frequency (especially arbitrage trading) absolutely impossible to do without a service such as bitinstant, which is currently MIA

[throwaway344]

Assuming that you're willing to maintain inventory at multiple exchanges, you could still absolutely do arbitrage inter-exchange.

[aianus]

Unfortunately there's no way to hedge your BTC inventory so you have to take on a lot of currency risk.

[bmelton]

Just so that it's said, buying at Coinbase right now gets you the right now price (or at least their spot price at the moment).

You're perfectly right that you won't receive the coin for 4 or more days, which leaves a lot of opportunity on the table for turning it around quickly, and perhaps increases the risk non-negligibly, but buying is done at spot price, not receipt.

[newobj]

Where does this 4 days of transit come from?

[Zoomla]

once your funds are in, you can instantly exchange between crypto-currencies

[ye]

Nothing stops you from doing HFT on one exchange, or on multiple ones independently. Most of them have APIs.