Nice one. From what I can tell, when you're outside the US, Google automatically redirects you to the local country. For example, here in Canada they redirect you to Google.ca. There's a link that says "Go to Google.com" near the "bottom" of the page.
I access Google via google.nl, google.com (in the Netherlands and in Kazakhstan) and via a US based proxy on a regular base.
The google.com I see in the Netherlands and Kazakhstan is different from the google.com I see in the US. Especially sites from outside the US seem to rank somewhat lower on the US google.com than on the international accesible google.com. I have not noticed any difference between the SERPs from google.com when I query from the Netherlands (Europe) or Kazakhstan (Asia). So it seems to me that the international google.com SERPs are independent of the location outside the US where you request them.
So if you want to know your actual ranking in the US, you need to access the search engine via a proxy.