If you have root explorer, open it up and navigate to:
1. /system/data/local/tmp and delete the rageagainstthecage file.
2. navigate to /system/app and delete Superuser.apk
3. navigate to /system/bin and deleate su and busybox
4. reboot the phone and you are no longer rooted.
Or you can use use the Android SDK and adb.
get into a shell "adb shell" you will have the # prompt
rm /system/data/local/tmp/rage*
rm /system/app/Superuser.apk
rm /system/bin/su
rm /system/bin/busybox
reboot