I had the same problem upgrading from 4.5 to 4.6.  For some reason the X 
server was not suid upon install.

You need to chmod +s /usr/X11R6/bin/NAMEOFSERVER

This worked for me, YMMV.

Gary

On Mon, 8 Jul 2002, Gilbert T. Gutierrez, Jr. wrote:
> I am having a problem with X and FreeBSD 4.6.  It works fine as root but
> when I am any other user it has a fatal error.
> 
> I get the following error when I run the command startx.
> Fatal server error:
> xf86OpenConsole: Server must be suid root