I wish I could help you both, but it is weird that z4 worked for some people after the update and not others. Did you update OTA or by downloading the update.zip? Did you make any system changes that didn't get reversed (like removing bloat without reinstalling, or freeze apps/change file names without unfreezing/renaming) before before updating? Had you been running any of the leaked updates before getting the official updates? Do you flash ROMS/themes? What about any overclocking/undervolting? Did you download z4root from the market when it was available or get it somewhere else?
There has to be some reason it works for some & not others.
I know it doesn't help but I can only speak from my experience. I had always kept my phone stock. I updated from 2.1>2.2 OTA when it was available. I rooted with z4root when I got it from the market about a month ago. I only rooted to run root apps. I never removed any bloat (other than NFS Shift that came with Froyo and wasn't blocked from being uninstalled the usual way). I have never ran any leaked updates. I was one of the few that got the 2.2.1 update early Wednesday morning. I didn't unroot or uninstall any root apps before I updated. The update did cause me to lose root, but it very quickly was able to reroot using the same z4root that I already had in my app list from when I initially downloaded it in the market before it was pulled.