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...