Or if you have ip dhcp snooping enabled, its easy:

sh ip dhcp snooping binding

I do it the way Jon describes. It works great.

Also a tip, if you're trying to find out where other switches connect to your core (assuming you're doing this on the core), you can do a sh int to find the mac address of that switchport, and then look for that mac address on the other switch to find out what port it connects to. Works well. I just had to map out my network doing it this way.

--John