I have a Lenovo laptop which has for a while had problems with the onboard wireless card, so I have been using a usb wireless device instead. I don't use wifi very often as I mostly have it connected to an ethernet cable, so although a bit annoying its not too much hardship having to use the usb device.
I recently put Ubuntu 11.04 on the laptop because the performance I was getting from Windows XP was deteriorating rapidly. I don't like the new Unity interface, chiefly because it forces you to have the panel on the left hand side, which is fine unless you use two monitors and you want your default monitor on the right, then you get the panel in the middle of your two screens instead of on one side. Unfortunately Mark Shuttleworth has insisted that It Must Be This Way so I can't see it changing any time soon, thus I'm sticking with the traditional Gnome2 interface.
Anyway, for whatever reason the network manager applet gets confused by the two network adaptors I have, and won't let me connect to a network on the usb adaptor, stating that its disabled in hardware. This is incorrect, its only the onboard network adaptor that has its wifi switch disabled (and in fact changing the switch doesn't make any difference to whether or not network manager thinks its disabled or not).
Running "rfkill list" confirms that the onboard adaptor is disabled in hardware, but the usb adaptor is not. The solution I found was to blacklist the modules used by the onboard adaptor. So I ran:
$ sudo vim /etc/modprobe.d/blacklist.conf
and I added the following lines:
blacklist iwlcore blacklist iwlagn
I was finally able to connect to wifi using the usb adaptor after also running:
$ sudo rmmod iwlcore iwlagn