How do you determine a mobile user's location?

[dogboy]

Ok, this is probably a loaded question, but imagine...

I'm surfing a standard .com website and it said, 'click this button and get the X/Y coordinates of your realtime position'. So I grant that app permissions to see my realtime location, and they give me my plottable location on the landing page. (If I was on my desktop, it would basically pin down my ISP's X/Y coordinates and if I was on my phone, it would know my exact location, give or take.)

...so the questions are, what technologies could that ('Checkin') site be employing? And (in rough/short terms) how would it work?

[grnidone]

This might shed some light: 
http://www.nytimes.com/2010/08/12/technology/personaltech/12basics.html

Also: 
http://icanstalku.com/