In a Google Groups thread, a merchant from the United Kingdom asks if he can bill his customers in USD rather than in GBP:
Can I bill in USD, rather than in GBP and they pay via a USD credit card?
Deborah, from the Google Checkout Team, responds with the answer:
If a customer places an order with a U.K. merchant, the purchase will be made in GBP. If a customer places an order with a U.S. merchant, the purchase will be made in USD.
If you have any further questions about this, you can feel free to respond in the Google Groups thread.