[update] A new XDA thread makes it even easier.
How to root the Vibrant
- Download the root (superuser, SU) package and rename it to update.zip
- Note: you may need to name the file update instead of "update.zip".
- On Mac OS X or Linux, you may also need to enable Settings : Applications : Development : USB debugging.
- This is the top level--not a subdirectory--of the 14gb drive (appears as E: in Windows). On the Android filesystem, the actual path will be /sdcard/update.zip
- If the USB cable is plugged into a computer, unplug it. Otherwise an interminable "Downloading..." screen will appear in the next step.
- Try holding volume-up with your left thumb, volume-down with your right thumb, then press the power button with your right index finger.
- Let go of the power button when you see the "Vibrant" start screen, or it might keep rebooting.
- At this point, if you see this error:
E: Can’t open /sdcard/update.zip(No such file or directory)Installation aborted.The path /sdcard/ refers to what you saw as the top level of your E: drive when you mounted your phone in Windows. Go back to step 1 or 4 and make sure update or update.zip is in the right place on your phone.
If you find that /system is mounted read-only, try this1 as root:
mount -o rw,remount yaffs2 /system
After deleting a .apk/.odex package via rm, you may need to also:
pm uninstall <package_name>
To mount system ro again:
mount -o remount,ro /system
Now that your phone is rooted, you may want to update the Superuser package to the latest version, which has many improvements.
How to un-root (that is, reverse the steps performed by update.zip)
Rooting may void your warranty or support with your carrier. In that case you might want to reverse the process. Here's how:
Note: this is only a partial reversal.
$ su # rm -r /system/app/Superuser.apk # rm -r /system/bin/su # rm -r /system/xbin/su # rm -r /system/xbin/busybox # reboot
 I assume you have installed a terminal app, and that you are familiar with the command line.