there is a thread over at the Arch forums: http://archlinuxarm.org/forum/viewtopic.php?f=31&t=2877
The user solved the problem by installing the package "xf86-video-fbdev". Could that be a solution for you, too?
It's a little different with him, though. Because his Pi seems to at least recognize the command.
Do you have "startx" in your bin or sbin folder?