I am sharing my iMac's ethernet connection as a WiFi hotspot. Who is connected? I would like to monitor who connects to my iMac's WiFi hotspot.  I have protected it with a password, but WEP is easy to crack. Is there a utility for OS 10.7.3/Lion to see who is connected?
arp -a
gives a list of connected interfaces on the same network.

en1 is Wi-Fi
en0 is Ethernet (on the Macbook Air this is Wi-Fi)

How to use
Type this command in the Terminal located at /Applications/Utilities/Terminal.app. To learn more about this command, type man arp in the terminal.