I use my iPhone on T-mobile (under contract until end of the month), after unlocking 1.1.4 with ZiPhone 3.0 back in March. Jealous of all the 2.0 excitement, I took the plunge and am in the middle of doing the hacked upgrade described here. Not sure if it was necessary, after ZiPhone hackage, but back on 1.1.4, I installed BootNeuter from installer.app and re-hacked that way. What I'm not clear on is what ZiPhone 3.0 actually did. As far as I can tell, my phone (an AT&T refurb) was already on the 3.6 bootloader, so I don't think anything wacky happened there. I'm a bit fuzzy on what exactly it was that ZiPhone was forced to downgrade - bootloader or baseband?
Regardless, my phone just rebooted after the 2.0 update and re-BootNeuter, and everything seems to be working fine. Kudos to the iPhone dev team!