As of a day ago you can get dollars out of Mt. Gox using Dwolla, which has been
excellent in my experience and only charges 25 cents per transaction. I withdrew money today and it was in my Dwolla account in less than an hour.
As far as using the API goes, there are still a few problems. My understanding from the #bitcoin-otc IRC channel is that there is a 30-second delay on trades on the Mt. Gox exchange. This contributes to weirdly huge sub-minute swings in the exchange rate. Two nights ago, there was a point where the rate went from $5 up to $6 and then back down to $5 within the space of a few seconds. This means that in the time it takes for the merchant to process the transaction, upload the bitcoins to the Mt. Gox exchange, and put in a sell order for the highest buy rate, the price may have changed dramatically.
Another problem is that the gap between the highest bid and the lowest asking price is often very large. There is no 'market rate' as of yet. So depending on when the merchant places a sell order, they may be getting a bad rate.
And it opens up another way for someone to manipulate the market to rip off the merchant. The highest buy order is usually placed by a bot that takes the highest non-bot buy order and increases it by something like .0001 bitcoins. The bots update their bids every minute or so. Since there are only a small number of traders in the exchange, a single large trader or group of medium-size traders could manipulate the market by placing a series of strategically high buy orders. This will drive up the overall rate, as the bots respond to the change in price by increasing their bids. (Edit: There are also bots programmed to make bids at somewhat less than the highest buy order, which would contribute to the illusion of a legitimate upward movement in the price, as all of those bots chase after the highest price.) Within just a few minutes, they can increase the highest bid by a significant amount (say $1 USD), and then place a bitcoin order for real goods and services with the merchant, knowing that they use an automatic system to exchange bitcoins for USD at the inflated market price. But during the time it takes the merchant to get their bitcoins into Mt. Gox's system and sell them, the price will have dropped back down as the fraudsters stop making inflated buy orders.