The command in the hint didnt' work for me in Mac OS X 10.6.8, but the one on the comments #1 and #2 did. Thanks

As to the use of networksetup for this, it's not really the same thing.
listpreferredwirelessnetworks is just that, the list of Preferred Networks that you have configured.

What the original hint is showing is the last time you've connected to any wireless SSID, regardless if you've marked it preferred or not.
Might help to narrow down a location if you pick up something nasty while mobile.

I've condensed all the grep/sed's down to one (equally incomprehensible) call to perl:

defaults read /Library/Preferences/SystemConfiguration/com.apple.airport.preferences KnownNetworks  | perl -lane 'print $1 if /SSID_STR.*= "?(.*?)"?;/ || /_timeStamp.*= "(\d{4}-\d\d-\d\d)/'