I use Arch Linux as the OS for several of my Raspberry Pies. There are times when you need to turn on wifi after Arch is installed on the SD card. Follow these steps to turn Wifi on:
Install the following 3 packages
$ sudo pacman -S wireless_tools wpa_supplicant dialog
Once the packages above as installed, you should be able to use a very nifty tool called:
$ sudo wifi-menu
The program will scan for networks and present you with a list.
$ sudo netctl enable <profile> $ sudo netctl start <profile>
Now you can safely unplug the network cable from the Pi and use the wifi network exclusively.