How do I see who's connected to my AP?

Issue the command:

wlanconfig ath0 list sta

Or just:

wlanconfig ath0 list

This should give you a nicely formatted list. See man wlanconfig 8 for more information.

If you are using Madwifi-Old

This is fairly simple, you just need to issue the command:

iwlist ath0 peers

Obviously modifying the ath0 where needed.

Since this command is regarded obsolete and will be removed from new wireless tools versions, you can also get the information (even more detailed) from following command:

cat /proc/sys/net/wlan/ath0/associated_sta