Different referring pages change display

Today I'm hijacking the blog for the sake of demonstrating something unusual to someone else. The following image shows two cropped views of the same page on About Chess.

The top image is a normal view of the page; the bottom image is a view of the page after I clicked through my RSS feed on MyYahoo (my.yahoo.com). The bottom view says, 'Welcome to About.com What is About.com? I'm Mark Weeks, your Guide to Chess. If this Yahoo! search is not what you were looking for, you may want to see our other popular topics, such as...'

The URL of the two views is the same. The server has detected that the visitor is coming from Yahoo.com and has changed the welcome message to mention Yahoo and to point to popular pages on the About Chess site. I like it!