why doesn't dhcp work in this case?
 

I have the following lan :

cable modem
-switch
--2 pc's windows xp/98
--wireless access point
---laptop with a wireless pcmcia card (D-link dwl 650+(xp/Mandrake 9.1))

I have to use dhcp because of my internet service provider, and I only have 3 publics IP's so the access point has a static one in order to not to assign one of 3 public addresses
All this works fine under windows

The problem is under linux because I can't use DHCP from my laptop.
The wireless card is properly configured because I have used it on another LAN using static IP and even I can do ping to the access point if I configure the wireles card using a static IP
I have also plugged the laptop right to the switch using laptop ethernet card and it works fine and I get internet connection using DHCP
Why can't I use DHCP with my wireless card?
(I use acx100sta module)

What can I do to get my wireless card working fine using DHCP?

Sounds like your Linux side is working, and your real question is about configuring your wireless hardware. Here is an alternative forum that might be more fruitful:
http://www.expansys.com/forum.asp?code=DWL-650PLUS