Well, we are looking for a new server for one of our larger sites. In doing so, I figured I would post an invaluable tool. This website will allow you to find out how fast other servers/places from around the planet are able to get to your IP, or datacenter. So, if you are looking at a new host, ask them for a test ip, and then start plugging that into some of the links on this page:
I suggest documenting some of the results. These tools will help you make a wise decision on where you server should be located at, and what to expect for latency.