Kelly,
When I started my online business, in order to get a merchant account and payment gateway, I had to have a Tax ID number. It was an F-EIN (Federal Employement Identification Number) I believe. Basically, you have to register your business with your state in order to get this number. Which also meant that I had to report my sales (on those that occured within the state of MO) to the IRS every quarter and pay sales tax on them...if there were any. (It is an online business so the sales are nationwide.)
I couldn't find any way around this and still be able to process major credit cards directly from my site. This was back before Google Checkout was around and still in the earlier days of Paypal. Now, one can always choose to use Google Checkout and Paypal exclusively and not bother with your own merchant account and gateway, but I think that limits your options in terms of methods of payment.
I would be interested to hear if anyone has any other alternatives to getting a payment gateway without registering the business.
Julia