Once again, digging through Apple's XML files has revealed the url to the iPhone 2.0 Firmware that is presently available on Apple's servers. The firmware image has a filename of:
/* Admin Edit: it's on Apple's server so we considre it released:
This build is slightly higher than the one of the latest iPhone Firmware Beta (5A345) which was also spotted on shipping iPhone 3Gs. Despite the filename appearing in the XML file, the actual firmware has not yet been released officially. As well, the firmware carriers a "1,2" designation while all previous firmwares, including the beta firmwares carried a "1,1" number.
Apple has posted an updated guide about firmware upgrades and warns:
Normally if you choose to update, the iPhone or iPod touch software is updated but your settings and media are not affected. If your device currently has a software version prior to 2.0 (1.x) and you are updating to software version 2.0 or later, all data on your device will be erased in order to perform install the new software. In this case, iTunes will offer to create a one-time media backup of your device depending on what content is on your device and what content is stored in the iTunes Library you are connected to. You should ensure that you have enough free space on your Mac or PC to accommodate a backup that matches the capacity of your iPhone or iPod touch (4 GB, 8 GB, 16 GB, or 32 GB) if you proceed with the this backup.
In order to install the firmware, you will have to do it manually by pressing the Option key when pressing the "Check for Update" button in iTunes.