We had the same problem with an Optiplex 760. See my lab mate Hunter's posting here:

http://community.qnx.com/sf/go/topc11177

Here is how I solved it:

Buy a PCI board with two PS2 and two USB outputs on it for around $20. (Boards with only PS2 outputs may also work but 
it is not tested).

Install the card in the PC, disable the USB controller in BIOS setup, boot from QNX RTOS bootable installation CD, 
connect original USB keyboard and mouse to the original USB ports of the computer (they will still be useful to get into
 BIOS settings), and connect a PS2 keyboard and a PS2 mouse to the PCI board.

When the installer asks for the license key, the PS2 keyboard should work to fill that in. Follow the steps of 
installation with this keyboard. After installation it boots. Here I ran into more problems about a "cannot attach mouse
 input report (error 2)" error. I tried safe mode (text mode) booting, which worked fine, but running photon would just 
hang the system with this error. At some point, I got it working, both keyboard and mouse worked in Photon, but with 
awful resolution. Trying to open display settings crashed the system (I think this is due to the fact that there is no 
driver for Intel Chipset 4 graphics card for QNX yet). When I was trying to solve this resolution issue by editing the 
display.conf file, Photon died and I was never able to get it working again. Only text mode worked. Then I did a fresh 
install from scratch, still was not able to go past the "error 2". Finally this worked: I disconnected PS2 keyboard and 
mouse and connected the USB keyboard and mouse to the USB ports on the PCI board (they are actually PS2 to USB 
converters), and then it worked, and I was able to get into Photon again. So what I advise you is, once the installation
 is complete, do the switch over from PS2 input devices to USB input devices, but of course with the ports on the PCI 
board, since all USB ports on the mother board are deactivated in BIOS settings (activating them again still hangs the 
system at startup). Still I got it working at some point partially out of luck and I am not sure why they worked (and 
for a while why they did not), but keep trying different combinations of the keyb and mouse and ports and it will 
eventually work. I am still seeing the "error 2" but it is not getting stuck there anymore.

Now the display settings still crashes the system. I just updated the resolution values in the display.conf file for the
 first two devices listed (vesa and i830) and it works with nice resolution now. Don't try to change the driver name 
(i830 or other) or you can get caught up in the "error 2".

Ozkan