Although this is already widely known in the SEO community, this article provides a neat summary of how results are country dependant. The TLD does not have much affect either, even though Google considers .us to be the same as .com – try going to google.us and see where it takes you. I wrote to Vint Serf about this but got no reply.
By the way, all web-tart hosting is UK based as this reflects its clients business base, despite the temptation of using incredibly cheap hosting from USA such as dreamhost. It just wouldn’t provide the right google search results for our clients.