I just got off the phone with Dell Support.  I had the exact same problem.  You'll need to reinstall your driver.  Right click on My Computer, click on Properties, Hardware, and then Device Manager.  Then click on the plus sign next to Network Adapters and then right click your wireless network connection (mine was Intel Pro/Wireless 2200BG).  Then click Unistall.  Then reboot your computer.  During the reboot your computer will automatically reinstall the driver.  Afterwards you might want to update your driver as well.  Mine worked without updating the driver.

I would like to recomend that you also go into the network device properties and make sure its transmit is set up to full and you might turn off the power saver to avoid the problem of no connection when on battery.