Not as easy as you'd think. There are a couple of easy ways to find out who's logged in to a linux machine: 'users' and 'who'. But I've discovered that when a user has been authenticated via ldap they don't show up when 'users' and 'who' are called. This MAY be a result of my configuration of the machine, but until that is determined, the only I've found of doing this is a bit convoluted. Run this one-liner when logged in to a remote machine:

me=`whoami`;today=`date +"%a %b %d"`;last|sort -r|grep "$today"|grep -v "$me"|grep -v reboot|cut -d " " -f1

Explanation
me=`whoami` -> we want to able to exclude ourselves from the output.
today=`date +"%a %b %d"` -> we need the current date, without the time
last -> the actual command we need to find users logged in with ldap
sort -r -> last outputs oldest last, this reverses the order
grep $today -> cleans up our output to only show stuff from today
grep -v "$me" -> exclude me from output
grep -v reboot -> exclude the last reboot command
cut -d " " -f1 -> get the first field of the output (this is the login name of the ldap user)

All of that and all you *really* get is the last logged in user. They may or may not actually be logged in right now.
I'll have to look in to my ldap config a bit more closely. There must be a way of including ldap auth'd users in the output from 'users' or 'who'.

NOTE: a simpler, possibly just-as-useful, method is to run this

last | grep 'still logged in' | grep 'tty' | cut -d " " -f 1

which returns the logged user running on a local display.