[Updated August 6 @ 10:15 AM EST]
Enterprising user rhcp0112345 over at XDA-Devs (where else?) has come up with an on-phone, one click root/unroot for the Samsung Galaxy S line of devices. It has only been tested on his Vibrant, but it should work across the line of devices.
There are no real details on the process yet - but it seems that you just install the app, hit “root” or “unroot,” and reboot your device.