I received an email this morning, informing me that I had been overcharged on the tax on my Nexus One. As such, Google is giving me a refund. It isn't just me either. It seems that everyone who purchased a Nexus One at or around launch is being notified.
Once Google sends you the email, you'll have to verify the card you want to receive the refund on by signing into your Google Checkout account and choosing to view the order. If the card you originally used is no longer valid, you'll be able to pick another. Google is also willing to mail you a check for the amount of the refund.
Kudos to Google for being proactive here. I'm not sure if they were tipped off about the tax issue or discovered it themselves. I never would have noticed on my own, and I doubt most other Nexus One owners would have either.