I know there are many posts like this, however none of the fixes I have found have worked.
I had the issue on 11.04, and after having no luck fixing it decided to try 12.04 however this has not fixed the problem. I'm using a Lenovo IdeaPad, the network card is a Atheros Communications AR9285.
edit add outputs:
sudo iwconfig lo no wireless extensions. wlan0 IEEE 802.11bgn ESSID:"NETGEAR-PLOW" Mode:Managed Frequency:2.437 GHz Access Point: E0:91:F5:7D:1B:BA Bit Rate=65 Mb/s Tx-Power=15 dBm Retry long limit:7 RTS thr:off Fragment thr:off Encryption key:off Power Management:on Link Quality=66/70 Signal level=-44 dBm Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0 Tx excessive retries:77 Invalid misc:63 Missed beacon:0 eth0 no wireless extensions. lspci -nnk | grep -iA2 net 06:00.0 Network controller : Atheros Communications Inc. AR9285 Wireless Network Adapter (PCI-Express) [168c:002b] (rev 01) Subsystem: Lenovo Device [17aa:30a1] Kernel driver in use: ath9k -- 07:00.0 Ethernet controller : Realtek Semiconductor Co., Ltd. RTL8101E/RTL8102E PCI Express Fast Ethernet controller [10ec:8136] (rev 02) Subsystem: Lenovo Device [17aa:392e] Kernel driver in use: r8169