We've always offered unlocked phones for direct sale to registered Android Developers. As of today, the Developer Phone is the Nexus One, at a price of $529. To see the details or order a phone, you need to sign in to your Android developer account and click on the "Development Phones" link.
The Nexus One combines an up-to-the-minute platform (Android 2.2), modern hardware, and the pure Android software suite. It's a good choice both for people who want to build Android applications using either the SDK or the NDK, and those who want to experiment with modified versions of the Android platform. Note that the Nexus One still ships with Android 2.1 but will download 2.2 soon after you turn it on; make sure you’re near a fast network.
As well as being an outstanding developer platform, it's a really nice everyday phone; we're really happy to have connected the right dots to make this happen.
[Update]: A bunch of people have spoken up wondering about Nexus One accessories. They are available right now in HTC's European online store. When we get more news, we'll pass it along.
[Update, Aug 6th]: The HTC US store now has accessories too.