See this pdf for more information.
If you were asking about a two currency arbitrage between different markets. For example, the BTC/USD exchange rate at MtGox and one at Intersango then the following formulas apply considering brokerage fees at both exchanges.
For simplicity I am using BTC as the base currency and USD as the quote currency, but you can exchange these easily.
A is the exchange rate of BTC/USD on exchange MtGox
B is the exchange rate of XXX/YYY on exchange Intersango
Fa is the fee of MtGox
Fb is the fee of Intersango
There is a no arbitrage opportunity if
A = B + ( Fa + Fb )
A - B > Fa + Fb then the the base currency, BTC, is cheaper on exchange B than in A. So the person would buy BTC on Intersango (B) and sell BTC on MtGox (A).
profit = A - B - ( Fa + Fb )
B - A > ( Fa + Fb ) then the base currency, BTC, is cheaper on exchange A than in B. So the person would buy the BTC on MtGox (A) and sell it on Intersango (B).
profit = B - A - ( Fa + Fb )
Other arbitrage opportunities will exist such as triangular arbitrage. But those won't develop until the exchanges not only have BTC/fiat currency pairs but other currency pairs as well.