If we consider arbitrage opportunities even between exchanges of one currency, for example if we take USD exchanges

it is very important to understand that to realize it we need to be able to execute two transactions at the same time.

One transation to sell some amount of BTC on one exchange and second transaction to buy the same amount of BTC on different exchange.

As we can see we need to pay attention not only to the bid and ask prices on corresponding exchanges but also to BTC/USD volume actually available on the corresponding exchanges.

Here is screenshot from bitcoin-analytics.com which demonstrates that at the moment there is an arbitrage opportunity between mtgoxUSD and btceUSD for the volume of 1000 BTC, and also between mtgoxUSD and cbxUSD.

For the mtgoxUSD/btceUSD difference is 101 USD if you execute such transaction

and 110 USD difference will be for mtgoxUSD/cbxUSD pair.

Also notice that composite arbitrage opportunity for both variants together allUSD will be 121 USD (if you split 1000 BTC and make two transactions together: one part with mtgoxUSD/btceUSD and also second part with mtgoxUSD/cbxUSD)

It is easy to imagine how this can be extended for multi currency arbitrage case.