*sigh*... unplugged all of my hard drives and plugged them back in again. Now it works.
Does xrandr show the second monitor? If it doesn't, it might be tricky to get it working. If it does show it, it's probably just a matter of setting up your xorg.conf correctly. You can play around with xrandr settings until you find some that work.
I'm trying to install Arch on an Asus Zenbook Prime UX31A. It already has Windows 7 x64 on it and a UEFI system partition doing the bootloading.I followed the steps for creating a UEFI bootable USB from the ISO.
This issue may be related to Syslinux 6.02, which has been identified and fix in the latest ISO. See https://bbs.archlinux.org/viewtopic.php?id=173665 and https://bbs.archlinux.org/viewtopic.php?id=172588 for further information.