Actually, I came up with a new test for this as follows:
1. Use a tabbed browser, and clear your cookies before running the test.
2. Go to google.com, and enter in a search term. I used, once again, "
seo web design" without quotes.
3. Click submit.
4. Open a new tab in your browser, and cut and paste the address from the SERPs page into the new tab. Verify you see the same results.
5. Open your command prompt. In WinXP, press windows+r and enter cmd. For Vista, type cmd in the Search bar at the bottom of your Start menu. If you have Linux, open your network control panel and find the ping utility.
6. Enter "ping google.com" at the command prompt.
7. You will see "Pinging google.com [x.x.x.x] with 32 bytes of data". x.x.x.x is the IP address of the Google datacenter you just ran your search on.
8. In the second tab's address bar, replace "www.google.com" with the IP address you got in step 7, and press enter.
9. The results will change, typically starting around position 5. Compare the two tabs.
I think this rules out localization factors and differences between data centers because both queries use the same data center. Simply put, it appears to me that Google serves different results or sorts results based on different parameters when you do a search directed at the IP address.
Because many tools that SEOs use to check rankings use the IP address of a specific datacenter rather than using the domain name, this can cause users of the tool to see different results from normal searchers.