WLAN on Ubuntu Gutsy Gibbon (7.10) with IPW3945 and WPA encryption

I updated from Feisty to Gutsy yesterday and had massive problems making an internet connection via wlan. My Acer Aspire 5650 has an Intel IPW3945 WLAN card and the kernel modules were loaded successfully without any errors. The restricted WLAN driver was listed as "in use" in Ubuntu's restriced driver manager, so I expected to have a working driver configuration.
Somehow I assumed it had something to do with the network manager. So I completely removed the Network Manager via

apt-get remove network-manager

and configured my wlan interface (using instructions from a german ubuntu forum) manually editing /etc/network/interfaces:

iface eth1 inet dhcp
        wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
auto eth1

My wpa_supplicant.conf looks something like this:

ctrl_interface=/var/run/wpa_supplicant
eapol_version=1
ap_scan=1

network={
        ssid="YourWlanSSID"
#        scan_ssid=1
        proto=WPA
        key_mgmt=WPA-PSK
        pairwise=TKIP
        group=TKIP
        psk=YourPSK
}

Note: I am using WPA encryption for my WLAN.
You can test your wpa_supplicant configuration using the following command:

sudo wpa_supplicant -i eth1 -D wext -c /etc/wpa_supplicant/wpa_supplicant.conf -d

Option -i defines the network interface name of your WLAN card. Option -D declares the driver to use and option -c defines the wpa_supplicant configuration file.
After this I restarted my system and I got a working wlan connection to my Access Point.