z4root won't work, are you using ADB or just the gingerbreak.apk?
Try it through ADB. It took a couple of times for it to take this last time, but I just rooted the other day using adb and the gingerbreak exploit.
just download this ziphttp://www.4shared.com/zip/MwKTjMkm/update.html
and place it into sdcard then turn off your phone (press home button >power>vulume) at the same time and scroll down to (wipe cache partition)press home button then scroll up to (apply sdcard:update.zip) and wait for it to do it things then its gonna ask you to reboot your phone adn u'll be find to try again to root or just keeit unrooted or try 4zroot it temporary root option...hope this help you ..peace out .