Is unroot still possible in this case? I have an LG Optimus One ( LGP500 ) running 2.2 and I rooted it using the z4root tool, version 1.0. From what I saw, back then it didn't have an unroot option. In the meantime, I did an update using the LG Mobile Tool, and I think it updated my system to 2.2.2 .

I noticed that the newer versions of z4root have an unroot option, so I upgraded my z4root from 1.0 to 1.3, to try to unroot the phone ... but I only see the Temporary root and Permanent root options. I tried pressing the Permanent root and it seemed to do something, after which it returned me to my Home. From this, I'm drawing the conclusion that rerooting did not work. I am assuming z4root needs to root, so that the unroot option is available.

Is there anyway I could unroot my phone now?
As you have come to realize, official updates (OTA or manual) will most likely patch previous exploits to obtain root and you will most likely lose root during the process of updating.  It is recommend to NOT allow updates if you wish to keep root.  Certain mods, such as CyanogenMod, will automatically block OTA updates to keep this from being a problem.