Thanks for the tip!

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.