When a new phone is released, a company would generally make an SDK available for it before the public can purchase one. This wasn't the case with the Nexus One, as it's been out for almost a week now, and Google has just released the new development tools.
Whilst you may be thinking that the new SDK would pack some nice changes to accommodate the new phone, you'd be wrong. Google has described the new Android 2.1 development kit as follows: "Android 2.1 is a minor platform release deployable to Android-powered handsets starting in January 2010. This release includes new API changes and bug fixes." According to Engadget, the real change for developers is the ability to produce your own active wallpapers, and everything else is simply aimed at patching up any issues.