I got my Model A Raspberry pi working with the Edimax EW-7811 USB wifi adapter and no additional network connectivity. I used a cheap USB hub and could power the mouse, keyboard, and wifi with no external power.
There are a few tricks: the first is to use the most recent version Raspbian.
The second trick is to edit the /etc/wpa_supplicant/wpa_supplicant.conf file and manually join to an SSID:
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
network={
ssid="luben"
proto=RSN
key_mgmt=WPA-PSK
pairwise=CCMP
group=CCMP
psk="*password*"
}
The /etc/network/interfaces file looks like this,
auto lo
iface lo inet loopback
iface eth0 inet dhcp
auto wlan0
allow-hotplug wlan0
iface wlan0 inet manual
wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp