Qt 5 does not work or has problems when using it from remove desktop clients like:
Xvnc, NX, Exceed on Demand and others.
In my case I use Exceed on Demand and keyboard does not work.

Even not using Qt from remote desktop this backward compatibility should allow to compile Qt 5.x.x with old versions of X compatible with Qt 4.x.x.

Even from my normal workstation with Red Hat 5 Qt 5.1.1 does not work, even having the X Keyboard extension enabled:
Setting verbose level to 10
locale is C
Applied rules from xorg:
model: pc104
layout: us
variant: intl
Trying to build keymap using the following components:
keycodes: xfree86+aliases(qwerty)
types: complete
compat: complete
symbols: pc(pc104)+us(intl)
geometry: pc(pc104)
xkb_keymap {
xkb_keycodes

{ include "xfree86+aliases(qwerty)" }

;
xkb_types

{ include "complete" };
xkb_compat { include "complete" }

;
xkb_symbols

{ include "pc(pc104)+us(intl)" }

;
xkb_geometry

{ include "pc(pc104)" }

;
};

Some related bugs:
https://bugreports.qt-project.org/browse/QTBUG-32760
https://bugreports.qt-project.org/browse/QTBUG-34483