XFree86 3.3.6 used to support the command "xset r rate" to set the keyboard repeat rate from within an X session. The current Rawhide XFree86 RPMs don't appear to support it, at least not by default, and I can't figure out how to enable it. Unless the functionality has been completely removed, which would be unfortunate, it should be enabled by default for backward compatibility.
This has been fixed as of XFree86-4.0-0.29.