I work for a large multinational in the human capital management space and we let a select number our customers do penetration testing. Our customers range from Fortune 500 to government agencies in the US and EU. It is not an unheard of practice, and I would argue it is quite common for these requests to come up, especially during contract negotiations.
My little firm can't afford stuff like that. So we outsource our testing to China and Russia - they charge a lot less.
Seems like they're always falling over each other to try and accommodate us.