Buy at one exchange, then sell at the other for instant profit!
This graph will help you to see where the most profit can be made. Keep this page open, these values change quite often.
It's difficult to get $ out of MtGox at present to do any hardcore arbitrage.
But if you already have money in both exchanges, you will be able to take advantage of the price difference by simply selling in one while buying in the other at the same time.
Latest Data (includes fees)
If you think it's worth ten cents, throw in a dollar :)