The other day, I wanted to buy a TV show from the US iTunes Store that was not available in my local (Swedish) iTunes Store.
There are several guides out there, but I didn't find one that explained payment well. Making a new post about it will also indicate that this solution worked fine at this time, August 2009.
The first step is to create a US account. Follow Apple's own tutorial on how to create an iTunes App Store account without a credit card.
Basically you switch the store settings to the US Store, start purchasing a free iPhone app such as Remote (iTunes Store link) and create an account with "None" as the payment option.
You need to enter US address details, such as those of any hotel in the USA.
The iTunes Store handles plus-addressing fine, so assuming your mail service supports it, you can use
If you don't have a US credit card, you can still use a gift certificate purchased by someone else in the US Store.
The iTunes Store has gift cards as well as gift certificates. gift cards are physical cards: to my knowledge you will not receive them by email. Gift certificates, on the other hand, are sent immediately by email.
So if you have a contact in the US, have them email you a gift certificate (iTunes Store link).
There are also gift cards available on eBay, and some sellers state they will email you the code. I have no experience with this. Their price will be a few dollars above the face value.
I have not tried this with anything I sync to the iPhone. This discussion thread suggests that you can sync to the iPhone from multiple iTunes Store accounts as long as the computer is authorized for all of them.
The thread also states that only the last account your computer was signed into will be available to the iTunes Music Store and the App Store on the iPhone.