Achernar - 2:37 pm on Mar 26, 2008 (gmt 0)

It depend on what information the browser shows to javascript.
Their method is also pretty basic.

On the same front you have browser detection. I've seen most sites, even google, using basic detection that can be spoofed easily by simply changing the user-agent. In fact there are alternative methods to better detect the type of browser. So far, the code I use hasn't reported false positives (100% reliable for FF, IE, Op). I'm sure a solid method can also be devised for OS detection.