Intersango already does this.
Intersango just reduces the amount they charge liquidity providers and increase the commission that liquidity takers pay. At Intersango, right now
Liquidity Providers PAY 0.35%
Liquidity Takers PAY 0.95%
The only exchange that I know of that actually gives a net rebate iswww.bitfloor.com
Liquidity Providers RECEIVE 0.10% and
Liquidity Takers PAY 0.4%
I trade at BitFloor and it's got a lot going for it. Deposit USD via Dwolla or Mt Gox code. Withdraw USD via Dwolla. Check it out.