It's not like I actually did anything except follow directions and click a few buttons, so take this as a public service announcement and not at all as a declaration of my own personal triumph: I successfully upgraded my-first gen iPhone to the UNLOCKED and JAILBROKEN iPhone software 2.0 thanks to the awesome PwnageTool created by the iphone-dev team.
After download Apple's official software update and the PwnageTool itself, the whole process took less than ten minutes - including a few PwnageTool-guided Google searches & downloads of bootloaders I didn't have on my laptop. The tool created a custom version of Apple's software and then guided me back to iTunes for installation. After install and reboot my iPhone flashed its own firmware according to PwnageTool's instructions and that was that. I'm now back up and running on T-Mobile with access to the App Store and Cydia (for installing Jailbreak apps).
So far, so good, and I'll report back with more as I go. Thanks, iPhone-dev team! You gals and guys PWN.