Today's the big day, and iPhone OS 3.0 is already showing up on Apple's servers. The update isn't seeding through iTunes yet, but you can download a restore image directly from the source.
The download is available here. (Warning: Direct link to the IPSW restore image)
Some notes: • To install this, you'll have to restore your iPod to the downloaded firmware. • The build number is the same a the Golden Master, which is to be expected. Likewise, the feature set should be the same. • This will not work on iPod Touches, so don't bother trying • The update will probably propagate through iTunes before too long
We'll keep you updated as the situation changes; for now, start exploring. [Many thanks, Kirk and Julie!]