What makes you think this is IP address based? I'm often outside the US and I've once had a survey dump me out that I can tell was IP address based. (... because it worked when I bounced the access through a US proxy instead.)

That said, I reckon it's worth setting up a proxy on a home machine (e.g. a well-secured linux box) anyhow if you're outside the US a lot. There are tons of US sites (e.g. TV networks) which only allow access from US IP addresses.