We're setting up payment processing on our site. We don't need anything fancy. Google has anti-competitively good pricing; it's free for the seller through 2008.
Perhaps there is something more professional seeming about an authorize.net style merchant account, but it's easier for us if we don't have to handle any credit card information. How much does using Google Checkout hurt us?
So if you want people to visit and order from your site, don't put any obstacles in their way. Whatever you do, don't force visitors to register."-Paul Graham, "The 10 Secrets of Selling Online: #4. Make Your Site Easy," http://store.yahoo.com/easy.html
By accepting only Google Checkout, you are adding an obstacle to everyone who does not already have a Google Checkout account, and thus discouraging casual browsers from executing a purchase. I'd personally encounter a lot less "mental friction" logging into an existing Paypal account or the like and pressing a few buttons than going to another site, starting creating an account, finding and typing in a credit-card number, enter an address, glance through the Terms of Service, etc...
Of course, depending on your product, there may not be such thing as a "casual browser" who may leave and look at a similar product merely based on the inconvenience of making an account, but it is nevertheless a large obstacle; it's just that the seeker and prize on either side have suddenly increased in strength.