I had a similar problem with a Linksys WUSB600N USB adapter.
The adapter was working (LED blinking) and it could scan all WiFi networks in the neighborhood. All seems in OK, except that I could not connect to any network with Wicd with DHCP or static IP (in the case of DHCP, Wicd gave the similar response "connection failed: unable to get IP address"). The problem was finally solved by blacklisting several generic drivers loaded by Ubuntu:
(see http://www.linuxquestions.org/questi...driver-622449/ for the full procedure that solved my problem).
May be this could be related to your problem.