I have nexus s with android 2.3.4. I want to upgrade to Latest available android version 3.2. When I go to settings - about phone -system updates, it gives me message your system is currently up to date.
Because your phone provider has not yet updated their repackaging of Android to that version. Each phone/tablet vendor customizes/tweaks Android for their device and only if they re-qualify and re-tweak a newer version will it show up as an update.
I think that if you jailbreak/root your phone you can install a plain vanilla Android OS, and thus upgrade to 3.2, however you might not have full functionality.
The Nexus S being a cellphone, it will most likely never run Android 3.0/3.1/3.2, which are meant for tablets. There's a certain possibility it will be able to run the next version -Ice Cream Sandwich-, though, whatever number that will end up being (if not by standard means, then via CyanogenMod).