The following install step failed: run preupgrade script for AppleMobileDeviceSupport. Contact the software manufacturer for assistance

C,

I ran into the same issue. Make sure your apple device in disconnected from the computer. It interrupts the itunes download. Disconnect and make sure itunes is closed, then try installing again. Remember that even though you close of itunes, it is often still running in the background. To make sure it's closed, click and hold the mouse on the itunes icon and select quit from the drop down menu. Then try installing the updated itunes.

Hope that helps.

SoEreal

Thanks for posting! It helped me out =-)

Thanks SoEreal. I did as you suggested. I opend Itunes and then Quit Itunes and the new Itunes 10.5 installed with flying colors. Thanks alot!