Only way to really profit from this is:
-10 pile large amounts of USD to MtGox (there are some fees involved)
-20 trade USD to BTC on MtGox
-30 trade BTC back to USD on bitfloor
-40 withdraw your USD from bitfloor (there are some fees involved)
-50 goto 10
Also note that steps 10 and 40 might have significant waiting times.
True, but right now if I were the guy who was on MtGox who has a sell order of 150 for $143, they could transfer it over to bitfloor and sell it for $145 and earn an extra $300 by selling through BitFloor. Aye, I suppose they'd need to cash out through bitfloor then, and who knows what that might be like.