When I was traveling in the UK I had a couple of really interesting experiences using the normal set of online services I use. The Internet really doesn't have a concept of countries, but I found myself unable to do some things while I was there because various online services made assumptions by mapping my IP address to a country. This is a process called geo targeting and it is used in online advertising almost always. My experience highlighted that you should be careful to use it for account settings or commerce transactions. For example, when I was in parts of the UK and tried to login to Google Reader I was greeted with this screen.
I'm not sure what language that is, but it isn't one I know. Now, while it wasn't english it was obvious to me, and I think to most people, that you should just login in the form provided. I figured once I logged in it would then know via my preferences where I live and flip the language back to english. Unfortunately, it wasn't so smart.
Another example of this problem popped up when I was trying to renew my Google Earth subscription. Unfortunately I wasn't even able to do it since it would only allow me to see the UK store and only allow me to buy the one in £'s even though my account is entirely setup in the US.
Good food for thought on how far you should take geo targeting in building out products, versus marketing.