It is possible to get the Realtek RTL8188CUS (EdiMax EW-7811Un) device to work properly under Arch Linux by using a later kernel. It seems according to this discussion that it's been working since kernel-3.13, but the default kernel that is currently installed by Arch Linux is version 3.12.26 (as of 5sept14). So to install a newer kernel you need to install the
linux-raspberrypi-latest package (currently 3.16.1):
pacman -S linux-raspberrypi-latest
Then reboot and you should have a system that talks to the Wifi using nl80211 and hence commands such as iw and hostapd will now work properly.
EDIT[14feb15]: Unfortunately the 'linux-raspberrypi-latest' package appears to have been discontinued. The latest Arch kernel now uses the Realtek driver (v4.0.2_9000.20130911), as opposed to the upstream-linux (rtlwifi) driver, which is more stable for normal use but does not provide for monitor mode. If you want to use the monitor mode driver you'll have to build the (rtlwifi) kernel module. Buliding the module isn't too hard - it's simplest to cross-compile the whole kernel on another Linux box. To enable the rtlwifi drivers you'll need to uncomment them from the raspberrypi kernel wireless config (Kconfig and Makefile) then enable rtlwifi when you run make menuconfig.