Although not officially released as of yet, Mac Rumors has discovered the download link for the iPhone 2.0 firmware.

The firmware which is version 5A347 is available to download here.

Apple has also recently posted a guide about updating the firmware on a iPhone or iPod touch, the upgrade to 2.0 comes with a warning:

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.

As the update isn’t officially available those wishing to update should heed caution. The firmware can be installed by holding the “Option” key and clicking the “Check for update” button in iTunes.