Is there a reason you couldn't involve more than one payment option?
Many US users would probably prefer to be paid either via paypal or paper check drawn on a US bank. International users won't like checks because the fees associated with them can be upwards of $70 per check in some places. International users are less comfortable with paypal since it has a negative history, and might prefer something like moneybookers.
If you're paying users larger amounts of money, you might look into reloadable prepaid debit cards. Chase, GE, and Citi, and Payoneer (a new entrant) all offer them, and the ones with the visa logo can be used pretty conveniently internationally for both merchandise and atms. This was how Google has paid their Summer of Code students for the past two years (last year was GE, this year it was Citi). The SoC program attracts an extremely wide international audience, so if the payment method works for them, I would seriously consider it.
One other option I haven't seen mentioned yet is Amazon Payments. They don't have a lot of marketshare yet, but their product is awesome, and a great many people have amazon accounts.
It's not entirely clear from the ad copy on this site, but you can use the flexpay API to send money to users as well as collect it.
http://aws.amazon.com/fps/
Amazon's international presence is pretty good, and their reputation is a lot better than paypal.