If you get a blank screen for the Superuser app for some reason or “Permission denied” error, you can re-install the su
file like this:
mount -o rw,remount -t yaffs2 /dev/block/mtdblock3 /system
cp /sdcard/su /system/bin/su
chmod 4755 /system/bin/su
You should get a Superuser app pop-up on your phone now.
This is what happened to me while I was rooting my own Droid X but by erasing the su app and copying it again solved it. Source: Droidx.net