It is non-trivial to get dollars into accounts on exchanges. It is also not an instant transfer of bitcoins from one exchange to another. your two step process really is a four step one:
0. Get dollars to Bitstamp to buy bitcoins
1. Buy bitcouns from Bitstamp
2. Transfer bitcoins from bitstamp to bitfloor
3. sell bitcoins on bitfloor (assuming the price hasn't changed).
What you are describing is "location arbitrage" (exploiting differing prices for the same goods in different locations). and it is one of the ways that prices get moved back into line.
With a bit of investment you could have some funds in all the exchanges and thus you could buy low on one exchange and sell high on another within milliseconds. Then you just balance the funds in the background to prevent running out of some type of resource on an exchange, but it doesn't have to be instant because you already have all types of funds on all the exchanges anyway.
Well, in order for this to work you'd have to have a BTC position in the exchange that's quoting higher in the first place. You would have to have entered that position at a price that is lower than your planned exit price in order for this to be profitable. You're still just as susceptible to volatility.
