Plug your SD-card into another Linux-pc.
$ sudo mount /dev/sda1 /media/sda1 # replace /dev/sda1 with your card. /media/sda1 may have to be created first
$ sudo rm /media/sda1/config.txt # removes the configuration file. It will be re-created with correct settings on next boot
$ sudo sync # writes any changes left to your card
Unplug your card, plug it into your pi an fire it up. Now the HDMI output should be working. It seems to be an Arch only issue but as you can see it can be easily solved as long as you know where to start
(sorry for any mistakes in spelling and grammar. English is not my native language)