In my case, the wireless network cards not recognized by Ubuntu were manufactured by LinkSys. So after searching Google I discovered that many wireless cards do not have Linux drivers. The good news is I found an application called Ndiswrapper which allows you to run windows wireless card drivers on Ubuntu.
Read it here
In a recent post, I discussed the initial experience of using Ubuntu for office work. One of the issues which I encountered was that some wireless network cards were not recognized by Ubuntu.