I think that it must be a driver issue, and how the driver "talks" to the OS about how to communicate with the card.
Some references to a similar problem I have found are :
http://my.ebay.com.au/ws/eBayISAPI.dll?MyEbay&gbh=1&_trksid=m38
+"But every time I start the Realtek USB WLAN Client Utility, I get the message "NOTICE: Please enable the WLAN card in the Network item of System Preferences". Under network preferences, I don't see the WLAN card on the left. Instead, I only see "Ethernet" and "Built-in Serial Port (1)". I made sure that the wifi was enabled in the bios. Any help/suggestions??"+
. . .and from another thread . . .
http://www.insanelymac.com/forum/lofiversion/index.php/t29797-50.html
+Jan 18 2008, 06:04 AM+
+I installed the latest drivers, but when I boot, it says "Please Enable the WLAN Card" it says it should be under system preferences in "network"+
+But I don't see how to enable any wireless device under the new Leopard (10.5.1) Network Preferences pane. wtfff?+
pippodark
+Jan 18 2008, 01:16 PM+
+In preferences Network, click on + and you can see ALL ethernet that you have, enable ALL ethernet, do not touch anything, restart the PC.+
+On restart you can see the realteck connected, at restart Leo or tiger use the DHCP but after you can change what you like.+
+This is an mistake of Apple, that Does not recognize the USB network card+
aphextom9
+Jan 22 2008, 01:38 AM+
+looks like that worked...haven't been connected for long though, we'll see...+
aphextom9
I tried a fresh installation, and rebooting after each step, but still no change to my system reporting that the WiFi card is an Ethernet connection and the Realtek WLAN Client Utility giving the message "Please enable the WLAN card in the Network item of System Preferences"