This Geeklet displays the current wireless network. I'm not sure if this is the most efficient way to do it, so if anyone else can think of a better command, let me know. It seems to take a little while to execute, so keep the refresh time quite high unless you want to cause performance problems.
Command:
system_profiler SPAirPortDataType | tail -9 | head -1 | sed -e "s/://" | sed -e 's/^[ t]*//'
Font: Lucida Grande
Colour: White
Size: 18
