It’s a very valid question this is raised over on Slashdot, and in my two experiences (only because the laptop was still under warranty, otherwise I wouldn’t have wasted my time) they don’t do very much.
The first time, the guy was able to run a diagnostics CD and then ship the laptop back to Sony (the touchpad had died).
The second time, the guy was able to look up the error message on Google, and then managed to break the display before he was able to run the diagnostics CD. Then he went on guessing what the problem was (including a program that had been uninstalled over a year ago, but had left registry entries behind).
Sadly, the Geek Squad can’t even live up to it’s name. There’s no geeks that work there, just losers who finished the 3-hour in-store training course. Don’t waste your time with them if you don’t have to, as several others have already found out. There are plenty of other qualified computer technicians out there, so open up your yellow pages and get someone who can actually help you.