adb push and mount problems
First, make sure adbd is running as root:

adb root

If you get an error that says something like "unable to run as root on production builds" it means that you must always issue su command in the adb shell to gain superuser privileges.  You will need to enter the shell before any mount commands will work.  You will need to push the file to the SD Card, then mount /system partition as read-write, and then copy the file to the /xbin directory like so:

adb push [path-to-iwconfig] /sdcard/
adb shell
su
mount -o remount,rw /system
cp /sdcard/iwconfig /system/xbin/
mount -o remount,ro /system
exit
exit