Many gateways prefer the low-risk approach of one-for-one redemption and will simply allow the GatewayUSD to float against BitUSD with a small but variable spread in the market. Users then pay a small variable conversion cost as they exit from BitUSD to fiat USD through GatewayUSD.
On the other hand, many users will want a direct conversion from BitUSD to fiat USD. In this mode of operation, the gateway takes care of providing all of the liquidity within a fixed percentage transaction fee. The gateways then compete on offering the lowest possible spread.
Once this happens, BitUSD is effectively as good as USD with a small fixed conversion fee. This fee will likely be no more than the withdraw and deposit fees that current exchanges charge. At that point, BitShares will be a fully operational exchange with many banking partners and no limits. At no point in time will user deposits ever be subject to default or confiscation by an exchange or gateway. A truly decentralized exchange will have been realized, and the original vision of BitShares completed.