Obviously, if you're getting an IP address, then the laptops are getting a network connection. If you were out of switch ports, you wouldn't have received an IP address at all from DHCP because DHCP would have been unreachable.
It is possible that these laptops had a DHCP lease already and are just keeping the same lease when they can't reach the DHCP server. The best way to test this would be to take the laptops into your switch room and plug them directly into the ports on your 3COM switch. If you have connectivity there, then you may want to have the wireing to the desk checked for possible short or something like that.
While you're in the switch room, see if you can add them to your network from there.
Good luck,