The market rate has been steadily narrowing between MtGox and BitStamp over the past few weeks.
The current difference is <1%.
This seems to indicate there's not much movement of capital between the two exchanges as it costs at least 2% to rotate capital between these two exchanges.
It takes time to rotate cash, Bitcoins on the other hand...
One week ago the difference was >$4 for several hours. I bought ~50BTC $104 at Bitstamp when it was $108@Mtgox.
But I guess these opportunities will get fewer as the arbitrators learns their lessons...
Yes, it's free to transfer BTC from Bitstamp to MtGox and you can buy low and sell high, but that's just a one-time event unless you rotate fiat back to Bitstamp to purchase BTC again.
That whole process incurs transaction fees and hence why there's a difference between MtGox and Bitstamp.
When the difference is less than the transaction fees though it indicates people are not rotating capital.