This thread is timely for me too - I just asked in the wrong forum. The above trick only works if you don't have a national Google. Here in .au I get the .au ads via www-sj too.
Upon closer inspection, I notice that google.de does not give me .au Adwords but I don't get US-only ones either. I only get international ones and .de ones. Ditto for google.nl
So the conclusion is that for the English language, we're forced to see the local ones.
Before I get an Anonymizer account, does anyone here know that I am assured of a US IP address? The free one based in .ws actually shows adwords for Mexico. (Yes, I will ask Anonymizer as well but someone here might have personal experience)