I would support this idea in concert with some sort of decentralized exchange.
MtGox can then issue MtGoxUSD colored coins, which get exchanged for BTC, and can be redeemed at MtGox later on by their new owner. MtGox would just be a broker, and so would the rest of the current exchanges. The decentralized exchange would handle the trade of colored coins for BTC, and work through some sort of publicly auditable blockchain.
Bitstamp already supports this, it is called ripple...