If your Optimus S has the older software version LS670ZV
C ("ZV
C") then you can use the Quicksettings to give your phone free WiFi Hotspot capability, as described in the link from Telemike above. If you have software version LS670ZV
D ("ZV
D") or later then Quicksettings will not work and you will have to root your phone, and install the ZVC ROM (system software) or a custom ROM in order to get WiFi Hotspot capability.
If you are willing to root your phone, there is a bunch of information at
this link, but the amount of information is a bit overwhelming, so I might suggest scrolling down that link until you get to my post, where I distill it down into the 2-3 links that you need to read in order to root your phone.
I guess it is also possible to
tether via USB, though i haven't tried it. (I refused to accept the ZVD software update so that I could keep my WiFi Hotspot capability.)
When my 3G signal is strong, I get about a 500-600kbps data connection. It's good enough for occasionally surfing the web when I'm out of the house-- especially since its "free". Personally I would not want to live with such a slow connection speed as my primary Internet connection at home, but it's probably plenty good enough for some people and the price is right.